OpenStack is dropping the py2.7 support in ussuri cycle.
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 section from setup.cfg: Wheel is not needed for python
3 only repo
- Remove ancient sections from setup.cfg
- Update requirements
- Update api-ref/source/conf.py for current openstackdocstheme
- Don't test for broken po files, this is done in the infra scripts
since a long time
- Change shebang lines to use python3
Change-Id: If5276a47b8822c5c19d045fbc5c41394f4a9dd2d