⬆️ Bump attrs from 21.2.0 to 21.3.0 (#18)

Bumps [attrs](https://github.com/python-attrs/attrs) from 21.2.0 to 21.3.0.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://github.com/python-attrs/attrs/releases">attrs's releases</a>.</em></p>
<blockquote>
<h2>21.3.0</h2>
<p>This is a big release in the history of <code>attrs</code> and finishes an arc that took way too long and also delayed this very overdue release. But it's done: <code>import attrs</code> that has been talked about for years[^issue], but fell victim to “just this one more thing” has <em>finally</em> landed.</p>
<p>From now on, modern <code>attrs</code> code looks like this:</p>
<pre lang="python"><code>from attrs import define
<p><a href="https://github.com/define"><code>@​define</code></a>
class HelloWorld:
modern: bool = True
</code></pre></p>
<p>The <code>define</code>/<code>field</code> APIs have been around for over a year and were very popular, now the rest of the package followed suit. I'm very excited that <code>attrs</code> remains relevant and keeps evolving over now more than half a decade. If you're curious about some of the background, the docs now contain a short <a href="https://www.attrs.org/en/stable/names.html">explanation and history lesson</a>. As long as our users keep pushing us, we will keep pushing forward class generation in Python!</p>
<p>Big thanks to my <a href="https://github.com/sponsors/hynek/dashboard">GitHub Sponsors</a>, <a href="https://tidelift.com/subscription/pkg/pypi-attrs?utm_source=pypi-attrs&amp;utm_medium=referral&amp;utm_campaign=enterprise&amp;utm_term=repo">Tidelift subscribers</a>, and <a href="https://ko-fi.com/the_hynekl">Ko-fi buyers</a> that help me mustering the motivation for such long-running project!</p>
<hr />
<p>Since the release took so long, there's more highlights than we can enumerate here, we'd just like to point out a breaking change in the new APIs: converters now run on setting attributes by default. If this is causing problems to you, you can disable that behavior by setting <code>@define(on_setattr=[])</code>.</p>
<p>[^issue]:  I have an issue from 2018 that I wanted to &quot;come back the moment this lands&quot;.</p>
<h1>Full Changelog</h1>
<h2>Backward-incompatible Changes</h2>
<ul>
<li>
<p>When using <code>@define</code>, converters are now run by default when setting an attribute on an instance -- additionally to validators. I.e. the new default is <code>on_setattr=[attrs.setters.convert, attrs.setters.validate]</code>.</p>
<p>This is unfortunately a breaking change, but it was an oversight, impossible to raise a <code>DeprecationWarning</code> about, and it's better to fix it now while the APIs are very fresh with few users. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/835">#835</a>, <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/886">#886</a></p>
</li>
<li>
<p><code>import attrs</code> has finally landed! As of this release, you can finally import <code>attrs</code> using its proper name.</p>
<p>Not all names from the <code>attr</code> namespace have been transferred; most notably <code>attr.s</code> and <code>attr.ib</code> are missing. See <code>attrs.define</code> and <code>attrs.field</code> if you haven't seen our next-generation APIs yet. A more elaborate explanation can be found <a href="https://www.attrs.org/en/latest/names.html">On The Core API Names</a></p>
<p>This feature is at least for one release <strong>provisional</strong>. We don't <em>plan</em> on changing anything, but such a big change is unlikely to go perfectly on the first strike.</p>
<p>The API docs have been mostly updated, but it will be an ongoing effort to change everything to the new APIs. Please note that we have <strong>not</strong> moved -- or even removed -- anything from <code>attr</code>!</p>
<p>Please do report any bugs or documentation inconsistencies! <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/887">#887</a></p>
</li>
</ul>
<h2>Changes</h2>
<ul>
<li><code>attr.asdict(retain_collection_types=False)</code> (default) dumps collection-esque keys as tuples. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/646">#646</a>, <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/888">#888</a></li>
<li><code>__match_args__</code> are now generated to support Python 3.10's <a href="https://docs.python.org/3.10/whatsnew/3.10.html#pep-634-structural-pattern-matching">Structural Pattern Matching</a>. This can be controlled by the <code>match_args</code> argument to the class decorators on Python 3.10 and later. On older versions, it is never added and the argument is ignored. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/815">#815</a></li>
<li>If the class-level <em>on_setattr</em> is set to <code>attrs.setters.validate</code> (default in <code>@define</code> and <code>@mutable</code>) but no field defines a validator, pretend that it's not set. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/817">#817</a></li>
<li>The generated <code>__repr__</code> is significantly faster on Pythons with f-strings. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/819">#819</a></li>
<li>Attributes transformed via <code>field_transformer</code> are wrapped with <code>AttrsClass</code> again. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/824">#824</a></li>
<li>Generated source code is now cached more efficiently for identical classes. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/828">#828</a></li>
<li>Added <code>attrs.converters.to_bool()</code>. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/830">#830</a></li>
</ul>
<!-- raw HTML omitted -->
</blockquote>
<p>... (truncated)</p>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/python-attrs/attrs/blob/main/CHANGELOG.rst">attrs's changelog</a>.</em></p>
<blockquote>
<h2>21.3.0 (2021-12-28)</h2>
<p>Backward-incompatible Changes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^</p>
<ul>
<li>
<p>When using <code>@define</code>, converters are now run by default when setting an attribute on an instance -- additionally to validators.
I.e. the new default is <code>on_setattr=[attrs.setters.convert, attrs.setters.validate]</code>.</p>
<p>This is unfortunately a breaking change, but it was an oversight, impossible to raise a <code>DeprecationWarning</code> about, and it's better to fix it now while the APIs are very fresh with few users.
<code>[#835](python-attrs/attrs#835) &lt;https://github.com/python-attrs/attrs/issues/835&gt;</code><em>,
<code>[#886](python-attrs/attrs#886) &lt;https://github.com/python-attrs/attrs/issues/886&gt;</code></em></p>
</li>
<li>
<p><code>import attrs</code> has finally landed!
As of this release, you can finally import <code>attrs</code> using its proper name.</p>
<p>Not all names from the <code>attr</code> namespace have been transferred; most notably <code>attr.s</code> and <code>attr.ib</code> are missing.
See <code>attrs.define</code> and <code>attrs.field</code> if you haven't seen our next-generation APIs yet.
A more elaborate explanation can be found <code>On The Core API Names &lt;https://www.attrs.org/en/latest/names.html&gt;</code>_</p>
<p>This feature is at least for one release <strong>provisional</strong>.
We don't <em>plan</em> on changing anything, but such a big change is unlikely to go perfectly on the first strike.</p>
<p>The API docs have been mostly updated, but it will be an ongoing effort to change everything to the new APIs.
Please note that we have <strong>not</strong> moved -- or even removed -- anything from <code>attr</code>!</p>
<p>Please do report any bugs or documentation inconsistencies!
<code>[#887](python-attrs/attrs#887) &lt;https://github.com/python-attrs/attrs/issues/887&gt;</code>_</p>
</li>
</ul>
<p>Changes
^^^^^^^</p>
<ul>
<li><code>attr.asdict(retain_collection_types=False)</code> (default) dumps collection-esque keys as tuples.
<code>[#646](python-attrs/attrs#646) &lt;https://github.com/python-attrs/attrs/issues/646&gt;</code><em>,
<code>[#888](python-attrs/attrs#888) &lt;https://github.com/python-attrs/attrs/issues/888&gt;</code></em></li>
<li><code>__match_args__</code> are now generated to support Python 3.10's
<code>Structural Pattern Matching &lt;https://docs.python.org/3.10/whatsnew/3.10.html#pep-634-structural-pattern-matching&gt;</code><em>.
This can be controlled by the <code>match_args</code> argument to the class decorators on Python 3.10 and later.
On older versions, it is never added and the argument is ignored.
<code>[#815](python-attrs/attrs#815) &lt;https://github.com/python-attrs/attrs/issues/815&gt;</code></em></li>
<li>If the class-level <em>on_setattr</em> is set to <code>attrs.setters.validate</code> (default in <code>@define</code> and <code>@mutable</code>) but no field defines a validator, pretend that it's not set.
<code>[#817](python-attrs/attrs#817) &lt;https://github.com/python-attrs/attrs/issues/817&gt;</code>_</li>
<li>The generated <code>__repr__</code> is significantly faster on Pythons with f-strings.
<code>[#819](python-attrs/attrs#819) &lt;https://github.com/python-attrs/attrs/issues/819&gt;</code>_</li>
<li>Attributes transformed via <code>field_transformer</code> are wrapped with <code>AttrsClass</code> again.
<code>[#824](python-attrs/attrs#824) &lt;https://github.com/python-attrs/attrs/issues/824&gt;</code>_</li>
<li>Generated source code is now cached more efficiently for identical classes.
<code>[#828](python-attrs/attrs#828) &lt;https://github.com/python-attrs/attrs/issues/828&gt;</code>_</li>
<li>Added <code>attrs.converters.to_bool()</code>.
<code>[#830](python-attrs/attrs#830) &lt;https://github.com/python-attrs/attrs/issues/830&gt;</code>_</li>
</ul>
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</blockquote>
<p>... (truncated)</p>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/python-attrs/attrs/commit/dd26edd68e12879f716c6554f25d957af299b801"><code>dd26edd</code></a> Prepare 21.3.0</li>
<li><a href="https://github.com/python-attrs/attrs/commit/20bf4b6e54a75201b378cff8e6dd9521d2da28f1"><code>20bf4b6</code></a> Go over CONTRIBUTING.md</li>
<li><a href="https://github.com/python-attrs/attrs/commit/d528dd425980eff3f43b0e29b0ce4dc81ecd8d84"><code>d528dd4</code></a> Fix more links</li>
<li><a href="https://github.com/python-attrs/attrs/commit/fcfb5a692cc8c9f8fde8e39bbd2c5733a47fb1e7"><code>fcfb5a6</code></a> Last pass over changelogs</li>
<li><a href="https://github.com/python-attrs/attrs/commit/e09873485e14e9b11d5d590a55280894df367d92"><code>e098734</code></a> Add logo to PyPI description</li>
<li><a href="https://github.com/python-attrs/attrs/commit/26c0cef8e48bd131d062d45bdaa0c949d4a2d035"><code>26c0cef</code></a> Streamline workflow</li>
<li><a href="https://github.com/python-attrs/attrs/commit/3333e749781a107c829717f7bc0382d33b538b6e"><code>3333e74</code></a> Remove dead achor</li>
<li><a href="https://github.com/python-attrs/attrs/commit/2d77d83d4e3ceadbf4414da5963623a20564c415"><code>2d77d83</code></a> Fix dataclass_transform links</li>
<li><a href="https://github.com/python-attrs/attrs/commit/b4dc9b07c70c16848960da077fc7ac18fe5e9bc8"><code>b4dc9b0</code></a> Better 2.7 example</li>
<li><a href="https://github.com/python-attrs/attrs/commit/d4e32209dc5855796e57c2b08bdc1c1702d051ab"><code>d4e3220</code></a> Use attrs namespace throughout examples.rst</li>
<li>Additional commits viewable in <a href="https://github.com/python-attrs/attrs/compare/21.2.0...21.3.0">compare view</a></li>
</ul>
</details>
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