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
- 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

Change-Id: I5d63389fbc4588700e09b06db85b52e7aab4a219