Bump sphinx from 3.5.2 to 3.5.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.5.2 to 3.5.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](https://github.com/sphinx-doc/sphinx/commits)

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

docs/requirements.txt

@@ -1,4 +1,4 @@
 
-sphinx==3.5.2
+sphinx==3.5.3
 PyYAML==3.13
 sphinx-issues==1.2.0