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 and reno version
- Update hacking version to 3.0
- move python3 setting into toxenv, remove install_command and
move constraints into deps
Change-Id: Ibb42f8199eb4ca93e438e5e7ef5d4c4e0b35a738
54f2eb0904