Bump sphinx from 3.4.1 to 3.4.2 (aio-libs#5381)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.1 to 3.4.2.
<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 3.4.2 (released Jan 04, 2021)</h1>
<h2>Bugs fixed</h2>
<ul>
<li><a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8164">#8164</a>: autodoc: Classes that inherit mocked class are not documented</li>
<li><a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8602">#8602</a>: autodoc: The <code>autodoc-process-docstring</code> event is emitted to the
non-datadescriptors unexpectedly</li>
<li><a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8616">#8616</a>: autodoc: AttributeError is raised on non-class object is passed to
autoclass directive</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/83d5a323ef113294bb1c6e93f7752451668ed886"><code>83d5a32</code></a> Bump to 3.4.2 final</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/b59a48d4139801f46272530ca30fe321b1073564"><code>b59a48d</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8650">#8650</a> from tk0miya/update_release_checklist</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/b3f8bd1e3cff91753fa524ee8058b32608e33335"><code>b3f8bd1</code></a> doc: Quote URLs in release checklist</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/1346ddf3175c1a8e84db1be0d262a490f3b16df1"><code>1346ddf</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8635">#8635</a> from tk0miya/update_copyright</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/f9968594206e538f13fa1c27c065027f10d4ea27"><code>f996859</code></a> A happy new year!</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/5383846ced558d592795a162182cb37310ae9577"><code>5383846</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8622">#8622</a> from tk0miya/8616_AttributeError_for_non_class</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/1353a7b82f704c20492e9b17be4127c032fc11ad"><code>1353a7b</code></a> Merge branch '3.4.x' into 8616_AttributeError_for_non_class</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/1dd0cc8494e3320fa4fac70398ac044bc4604b87"><code>1dd0cc8</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8611">#8611</a> from tk0miya/8602_process-docstring_for_nondatadescr...</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/e3b1fdeeebeca6b7c03cc16dce5d9b686292a1ac"><code>e3b1fde</code></a> Merge branch '3.4.x' into 8602_process-docstring_for_nondatadescriptors</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/0f8debe55863e751ffa2c7ae064a43cf988f1d41"><code>0f8debe</code></a> Fix <a href="https://github-redirect.dependabot.com/sphinx-doc/sphinx/issues/8616">#8616</a>: autodoc: AttributeError when non-class is passed to autoclass</li>
<li>Additional commits viewable in <a href="https://github.com/sphinx-doc/sphinx/compare/v3.4.1...v3.4.2">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

@@ -1,6 +1,6 @@
 aiohttp-theme==0.1.6
 pygments==2.7.3
-sphinx==3.4.1
+sphinx==3.4.2
 sphinxcontrib-asyncio==0.3.0
 sphinxcontrib-blockdiag==2.0.0
 towncrier==19.2.0