Bump attrs from 20.2.0 to 20.3.0 (aio-libs#5207)

Bumps [attrs](https://github.com/python-attrs/attrs) from 20.2.0 to 20.3.0.
<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>20.3.0</h2>
<h2>Backward-incompatible Changes</h2>
<ul>
<li>
<p><code>attr.define()</code>, <code>attr.frozen()</code>, <code>attr.mutable()</code>, and <code>attr.field()</code> remain <strong>provisional</strong>.</p>
<p>This release does <strong>not</strong> change change anything about them and they are already used widely in production though.</p>
<p>If you wish to use them together with mypy, you can simply drop <a href="https://gist.github.com/hynek/1e3844d0c99e479e716169034b5fa963#file-attrs_ng_plugin-py">this plugin</a> into your project.</p>
<p>Feel free to provide feedback to them in the linked issue <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/668">#668</a>.</p>
<p>We will release the <code>attrs</code> namespace once we have the feeling that the APIs have properly settled. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/668">#668</a></p>
</li>
</ul>
<h2>Changes</h2>
<ul>
<li><code>attr.s()</code> now has a <em>field_transformer</em> hook that is called for all <code>Attribute</code>s and returns a (modified or updated) list of <code>Attribute</code> instances. <code>attr.asdict()</code> has a <em>value_serializer</em> hook that can change the way values are converted. Both hooks are meant to help with data (de-)serialization workflows. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/653">#653</a></li>
<li><code>kw_only=True</code> now works on Python 2. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/700">#700</a></li>
<li><code>raise from</code> now works on frozen classes on PyPy. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/703">#703</a>, <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/712">#712</a></li>
<li><code>attr.asdict()</code> and <code>attr.astuple()</code> now treat <code>frozenset</code>s like <code>set</code>s with regards to the <em>retain_collection_types</em> argument. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/704">#704</a></li>
<li>The type stubs for <code>attr.s()</code> and <code>attr.make_class()</code> are not missing the <em>collect_by_mro</em> argument anymore. <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/711">#711</a></li>
</ul>
</blockquote>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/python-attrs/attrs/blob/master/CHANGELOG.rst">attrs's changelog</a>.</em></p>
<blockquote>
<h2>20.3.0 (2020-11-05)</h2>
<p>Backward-incompatible Changes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^</p>
<ul>
<li>
<p><code>attr.define()</code>, <code>attr.frozen()</code>, <code>attr.mutable()</code>, and <code>attr.field()</code> remain <strong>provisional</strong>.</p>
<p>This release does <strong>not</strong> change change anything about them and they are already used widely in production though.</p>
<p>If you wish to use them together with mypy, you can simply drop <code>this plugin &lt;https://gist.github.com/hynek/1e3844d0c99e479e716169034b5fa963#file-attrs_ng_plugin-py&gt;</code>_ into your project.</p>
<p>Feel free to provide feedback to them in the linked issue <a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/668">#668</a>.</p>
<p>We will release the <code>attrs</code> namespace once we have the feeling that the APIs have properly settled.
<code>[aio-libs#668](python-attrs/attrs#668) &lt;https://github.com/python-attrs/attrs/issues/668&gt;</code>_</p>
</li>
</ul>
<p>Changes
^^^^^^^</p>
<ul>
<li><code>attr.s()</code> now has a <em>field_transformer</em> hook that is called for all <code>Attribute</code>\ s and returns a (modified or updated) list of <code>Attribute</code> instances.
<code>attr.asdict()</code> has a <em>value_serializer</em> hook that can change the way values are converted.
Both hooks are meant to help with data (de-)serialization workflows.
<code>[aio-libs#653](python-attrs/attrs#653) &lt;https://github.com/python-attrs/attrs/issues/653&gt;</code>_</li>
<li><code>kw_only=True</code> now works on Python 2.
<code>[aio-libs#700](python-attrs/attrs#700) &lt;https://github.com/python-attrs/attrs/issues/700&gt;</code>_</li>
<li><code>raise from</code> now works on frozen classes on PyPy.
<code>[aio-libs#703](python-attrs/attrs#703) &lt;https://github.com/python-attrs/attrs/issues/703&gt;</code><em>,
<code>[aio-libs#712](python-attrs/attrs#712) &lt;https://github.com/python-attrs/attrs/issues/712&gt;</code></em></li>
<li><code>attr.asdict()</code> and <code>attr.astuple()</code> now treat <code>frozenset</code>\ s like <code>set</code>\ s with regards to the <em>retain_collection_types</em> argument.
<code>[aio-libs#704](python-attrs/attrs#704) &lt;https://github.com/python-attrs/attrs/issues/704&gt;</code>_</li>
<li>The type stubs for <code>attr.s()</code> and <code>attr.make_class()</code> are not missing the <em>collect_by_mro</em> argument anymore.
<code>[aio-libs#711](python-attrs/attrs#711) &lt;https://github.com/python-attrs/attrs/issues/711&gt;</code>_</li>
</ul>
<hr />
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/python-attrs/attrs/commit/f3762ba07bf0cec8bebdaaeca2212ba1d662ff13"><code>f3762ba</code></a> Prepare 20.3.0</li>
<li><a href="https://github.com/python-attrs/attrs/commit/3d66e5727b93d3463feb0fd780ae739192e7c363"><code>3d66e57</code></a> Exclude GitHub issues from linkcheck to avoid rate limits</li>
<li><a href="https://github.com/python-attrs/attrs/commit/06d0f8eda63b3f08cb5d94fff9fddf4fe6695621"><code>06d0f8e</code></a> Add funding URLs to metadata</li>
<li><a href="https://github.com/python-attrs/attrs/commit/d23924f765d09f77e87a5c51c1685df0d721b469"><code>d23924f</code></a> Add provisional notice</li>
<li><a href="https://github.com/python-attrs/attrs/commit/cd2f886d6372dca288abf9d999b3e35dacd9ced6"><code>cd2f886</code></a> Use 'i' and 'k' to better distinguish variables in an example (<a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/713">#713</a>)</li>
<li><a href="https://github.com/python-attrs/attrs/commit/f2dabeae82c75abe174aafc7a726481ded929872"><code>f2dabea</code></a> Fix exception chaining on PyPy (<a href="https://github-redirect.dependabot.com/python-attrs/attrs/issues/712">#712</a>)</li>
<li><a href="https://github.com/python-attrs/attrs/commit/6b4a1f1ce65162afe54e7101b263859bf8b2177e"><code>6b4a1f1</code></a> Tighten up mypy configuration</li>
<li><a href="https://github.com/python-attrs/attrs/commit/9f7d11e415bc789c9cbab255cb9a5b8903c1b122"><code>9f7d11e</code></a> Add types to collect_by_mro annotations m(</li>
<li><a href="https://github.com/python-attrs/attrs/commit/7020c8b5b55adefc76cf369fb071b1a4b15554a9"><code>7020c8b</code></a> pre-commit autoupdate</li>
<li><a href="https://github.com/python-attrs/attrs/commit/56c73081c5d767e30f529ac4f15a6bdd43e23d9f"><code>56c7308</code></a> Add missing collect_by_mro to typing stubs</li>
<li>Additional commits viewable in <a href="https://github.com/python-attrs/attrs/compare/20.2.0...20.3.0">compare view</a></li>
</ul>
</details>
<br />


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requirements/base.txt

@@ -2,7 +2,7 @@
 # required c-ares will not build on windows and has build problems on Macos Python<3.7
 aiodns==2.0.0; sys_platform=="linux" or sys_platform=="darwin" and python_version>="3.7"
 async-timeout==4.0.0a3
-attrs==20.2.0
+attrs==20.3.0
 Brotli==1.0.9
 cchardet==2.1.7
 chardet==3.0.4