Bump sphinx from 3.3.0 to 3.3.1 (#545)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.3.0 to 3.3.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.3.0...v3.3.1)

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

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

requirements/doc.txt

@@ -1 +1 @@
-sphinx==3.3.0
+sphinx==3.3.1