Bump sphinx from 3.5.2 to 3.5.3 (#5553)

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[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/doc.txt

@@ -1,6 +1,6 @@
 aiohttp-theme==0.1.6
 pygments==2.8.1
-sphinx==3.5.2
+sphinx==3.5.3
 sphinxcontrib-asyncio==0.3.0
 sphinxcontrib-blockdiag==2.0.0
 towncrier==19.2.0