⬆️ Bump sphinx from 2.3.0 to 2.3.1 (#51)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.3.0 to 2.3.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.3.0...v2.3.1)

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

Co-authored-by: null <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

requirements_dev.txt

@@ -3,7 +3,7 @@ black==19.10b0
 blacken-docs==1.4.0
 pip==19.3.1
 pre-commit==1.20.0
-Sphinx==2.3.0
+Sphinx==2.3.1
 twine==3.1.1
 wheel==0.33.6
 yamllint==1.19.0