Update sphinx requirement from <3.1.0,>=3.0.0 to >=3.0.0,<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](https://github.com/sphinx-doc/sphinx/commits)

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

docs/requirements.txt

@@ -1,3 +1,3 @@
 sphinx-markdown-tables>=0.0.14,<0.1.0
 recommonmark>=0.6.0,<0.7.0
-sphinx>=3.0.0,<3.1.0
+sphinx>=3.0.0,<3.6.0