Bump sphinx from 2.2.1 to 3.1.2 (#274)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.2.1 to 3.1.2.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.2.1...v3.1.2)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

requirements-doc.txt

@@ -1,4 +1,4 @@
 setuptools>=38.0.0
-sphinx==2.2.1
+sphinx==3.1.2
 sphinxcontrib-asyncio==0.2.0
 aiohttp-theme==0.1.6