Bump sphinx from 2.4.1 to 3.5.4

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.4.1 to 3.5.4.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/4.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.4.1...v3.5.4)

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

requirements_dev.txt

@@ -9,7 +9,7 @@ pip==20.0.2
 plumbum==1.6.8
 pytest==5.3.5
 PyYAML==5.3
-Sphinx==2.4.1
+Sphinx==3.5.4
 tox==3.14.3
 wheel==0.34.2
 watchdog==0.10.2