Bump sphinx from 3.5.1 to 3.5.2 in /requirements/CI

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.5.1 to 3.5.2.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.5.1...v3.5.2)

Signed-off-by: dependabot[bot] <[email protected]>

requirements/CI/requirements.txt

@@ -5,7 +5,7 @@ pytest==6.2.2
 pytest-cov==2.11.1
 pytest-xdist==2.2.1
 setuptools-scm==5.0.2
-sphinx==3.5.1
+sphinx==3.5.2
 sphinx-argparse==0.2.5
 sphinx-issues==1.2.0
 sphinx_rtd_theme==0.5.1