Bump sphinx from 3.3.0 to 3.3.1 (#5223)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.3.0 to 3.3.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.3.0...v3.3.1)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/doc.txt

@@ -1,6 +1,6 @@
 aiohttp-theme==0.1.6
 pygments==2.7.2
-sphinx==3.3.0
+sphinx==3.3.1
 sphinxcontrib-asyncio==0.3.0
 sphinxcontrib-blockdiag==2.0.0
 towncrier==19.2.0