Bump sphinx from 3.4.2 to 3.4.3 (#561)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.2 to 3.4.3.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES">sphinx's changelog</a>.</em></p>
<blockquote>
<h1>Release 3.4.3 (released Jan 08, 2021)</h1>
<h2>Bugs fixed</h2>
<ul>
<li><a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8655">#8655</a>: autodoc: Failed to generate document if target module contains an
object that raises an exception on <code>hasattr()</code></li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/55cdadf973b89b3181407c478075ac0fc6984b3d"><code>55cdadf</code></a> Bump to 3.4.3 final</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/ac12d8dabe925cf96c1b9e696f9fa0e42999edac"><code>ac12d8d</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8656">#8656</a> from tk0miya/8655_exception_on_hasattr</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/a51c8a565280529e25d1cad879349d7addc1414d"><code>a51c8a5</code></a> Fix <a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8655">#8655</a>: autodoc: Crashes when object raises an exception on hasattr()</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/4755557a7df0a335b62c3f6bc062ce1c01c2d45a"><code>4755557</code></a> Bump version</li>
<li>See full diff in <a href="https://github.com/sphinx-doc/sphinx/compare/v3.4.2...v3.4.3">compare view</a></li>
</ul>
</details>
<br />


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Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting `@dependabot rebase`.

[//]: # (dependabot-automerge-start)
[//]: # (dependabot-automerge-end)

requirements/doc.txt

@@ -1,3 +1,3 @@
-sphinx==3.4.2
+sphinx==3.4.3
 pyenchant==3.2.0
 sphinxcontrib-spelling==7.1.0