Bump sphinx from 3.4.1 to 3.5.1

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.1 to 3.5.1.
- [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/v3.4.1...v3.5.1)

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

requirements-dev.txt

@@ -9,7 +9,7 @@ unittest2==1.1.0
 selenium==3.141.0
 twine==3.3.0
 Pallets-Sphinx-Themes==1.2.3
-Sphinx==3.4.1
+Sphinx==3.5.1
 sphinx-issues==1.2.0
 sphinx-jinja==1.1.1
 sphinx-tabs==2.0.1