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
- Remove old variables from conf.py, update openstackdocstheme
requirement.
- Remove hacking and friends from lower-constraints, they are
not needed in installed system.
Change-Id: I179d61545169f3d329c746c88dd4d38035d4075d
