Bump sphinx from 3.1.2 to 3.4.1

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.1.2 to 3.4.1.
- [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...v3.4.1)

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

docs-requirements.txt

@@ -1,5 +1,5 @@
 # Packages needed to build docs on https://readthedocs.org
 Django==2.2.13
-Sphinx==3.1.2
+Sphinx==3.4.1
 sphinx-autobuild==0.7.1
 sphinxcontrib-httpdomain==1.5.0