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
- Use newer openstackdocstheme and Sphinx versions
- Cleanup */source/conf.py to remove now obsolete content.
- Remove install_command from tox.ini, the default is fine
Change-Id: I623b1e0bfc2ec8cd9ce017556101cdeb06a1bb4c
d399c7dbe1