This repo is now testing only with Python 3, so let's 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
- Update classifiers
- Update requirements, no need for python_version anymore
- Use newer openstackdocstheme version
- Add basepython to tox.ini
Change-Id: I4b731354119ebc3a7519da420121404aa2fbb640