chore(deps-dev): bump sphinx from 3.5.0 to 3.5.1

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.5.0 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@v3.5.0...v3.5.1)

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

dev-requirements.txt

@@ -6,7 +6,7 @@ pytest==4.6.9;python_version<'3.0'
 pytest==6.2.2;python_version>='3.6'
 pytest-cov==2.11.1
 Sphinx==1.8.5;python_version<'3.0'
-Sphinx==3.5.0;python_version>='3.6'
+Sphinx==3.5.1;python_version>='3.6'
 sphinx-bootstrap-theme==0.7.1
 sphinxcontrib-websupport==1.1.2;python_version<'3.0'
 sphinxcontrib-websupport==1.2.4;python_version>='3.6'