Bump sphinx from 2.4.4 to 3.0.0

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.4.4 to 3.0.0.
- [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/v2.4.4...v3.0.0)

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

requirements-dev.txt

@@ -5,7 +5,7 @@ pytest==5.4.1
 pytest-cov==2.8.1
 pytest-mock==3.0.0
 coverage==5.0.4
-sphinx==2.4.4
+sphinx==3.0.0
 pep257==0.7.0
 aiohttp-session==2.9.0
 aiopg[sa]==1.0.0