Bump sphinx from 3.4.1 to 3.4.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.1 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](https://github.com/sphinx-doc/sphinx/compare/v3.4.1...v3.4.3)

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

requirements-dev.txt

@@ -5,7 +5,7 @@ pytest==6.2.1
 pytest-cov==2.10.1
 pytest-mock==3.4.0
 coverage==5.3.1
-sphinx==3.4.1
+sphinx==3.4.3
 pep257==0.7.0
 aiohttp-session==2.9.0
 aiopg[sa]==1.1.0