Bump sphinx from 3.5.3 to 3.5.4 (aio-libs#5607)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.5.3 to 3.5.4.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/4.x/CHANGES)
- [Commits](https://github.com/sphinx-doc/sphinx/commits/v3.5.4)

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==3.5.3
+sphinx==3.5.4
 sphinxcontrib-asyncio==0.3.0
 sphinxcontrib-blockdiag==2.0.0
 towncrier==21.3.0