Update sphinx requirement from ~=3.3 to ~=3.4

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [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...v3.4.0)

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

docs/requirements.txt

@@ -1 +1 @@
-sphinx~=3.3
+sphinx~=3.4