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>

Author: dependabot-preview[bot]

docs/requirements.txt

@@ -1,5 +1,5 @@
 marshmallow>=2.0.0rc1
 Flask==1.1.2
-sphinx==3.5.2
+sphinx==3.5.3
 sphinx-rtd-theme==0.5.0
 sphinx-issues>=0.2.0