Bump sphinx from 2.3.1 to 2.4.1 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.3.1 to 2.4.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/2.0/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.3.1...v2.4.1)

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

docs/requirements.txt

@@ -1,7 +1,7 @@
 # This file is used to setup env
 # to generate documentation.
 
-sphinx==2.3.1
+sphinx==2.4.1
 sphinx_autodoc_typehints==1.10.3
 recommonmark==0.6.0
 m2r==0.2.1