Bump sphinx from 2.4.4 to 3.1.0

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.4.4 to 3.1.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.4.4...v3.1.0)

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

requirements_dev.txt

@@ -7,7 +7,7 @@ watchdog==0.10.2
 flake8==3.7.9
 tox==3.14.6
 coverage==5.0.4
-Sphinx==2.4.4
+Sphinx==3.1.0
 twine==3.1.1
 pylint==2.4.4