chore(deps): bump sphinx from 2.2.1 to 2.4.3 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.2.1 to 2.4.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.2.1...v2.4.3)

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

docs/requirements.txt

@@ -3,7 +3,7 @@
 
 
 # dependencies
-sphinx==2.2.1
+sphinx==2.4.3
 jinja2==2.10.1
 git+https://github.com/jhermann/pygments-markdown-lexer.git@e651a9a3f664285b01451eb39232b1ad9af65956#egg=pygments-markdown-lexer
 pygments-apiblueprint==0.1.0