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
* Some other sections are obsolete
- Update classifiers
- Update requirements, no need for python_version anymore
- Remove html_last_updated_fmt from conf.py, this is not needed with
openstackdocstheme anymore; update openstackdocstheme requirement
Change-Id: I5389ba08c985e023643c07adf3135809c291146f