Bump sphinx from 3.1.2 to 3.3.1

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.1.2 to 3.3.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.1.2...v3.3.1)

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

requirements/docs_requirements.txt

@@ -1,4 +1,4 @@
-sphinx==3.1.2
+sphinx==3.3.1
 recommonmark==0.6.0
 sphinx_rtd_theme==0.5.0
 sphinx-markdown-tables==0.0.15