Bump sphinx from 3.4.2 to 3.4.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.2 to 3.4.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.4.2...v3.4.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -10,5 +10,5 @@ pytest-asyncio==0.14.0
 coveralls==2.2.0
 pycodestyle==2.6.0
 sphinx-rtd-theme==0.5.1
-sphinx==3.4.2
+sphinx==3.4.3
 mypy==0.790