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 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: I8811b249e53df8e25c46fe63d71b7dbb400dac19