Bump sphinx from 3.4.3 to 3.5.1

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.4.3 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](sphinx-doc/sphinx@v3.4.3...v3.5.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements_dev.txt

@@ -3,7 +3,7 @@ coverage==5.4
 flake8==3.8.4
 pip==21.0.1
 pytest==6.2.2
-Sphinx==3.4.3
+Sphinx==3.5.1
 tox==3.21.3
 twine==3.3.0
 wheel==0.36.2