Bump sphinx from 2.3.1 to 3.0.1

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.3.1 to 3.0.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.3.1...v3.0.1)

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

requirements/dev.txt

@@ -8,5 +8,5 @@ pytest==5.4.1 ;python_version > '3.0'
 pytest==4.6.9 ;python_version < '3.0'
 pytest-cov==2.8.1
 pytest-xdist==1.31.0
-Sphinx==2.3.1 ;python_version > '3.0'
+Sphinx==3.0.1 ;python_version > '3.0'
 Sphinx==1.8.5 ;python_version < '3.0'