Make a few cleanups:
- Remove python 2.7 stanza from setup.py
- Add requires on python >= 3.6 to setup.cfg so that pypi and pip
know about the requirement
- Remove obsolete sections from setup.cfg: Wheel is not needed for python
3 only repo
- Update requirements, no need for python_version anymore
- Use modern sphinx-build for docs building, update openstackdocstheme,
fix api-doc building.
Change-Id: I40b67f330ac850bf45cca742a8b967fd12480ce3
