Update sphinx requirement from <3.4.0,>=3.0.3 to >=3.0.3,<3.6.0

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@v3.0.3...v3.5.0)

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

requirements.txt

@@ -1,2 +1,2 @@
 docutils>=0.16
-sphinx<3.4.0,>=3.0.3
+sphinx>=3.0.3,<3.6.0