Make a few cleanups:
- Remove python 2.7 stanza from setup.py
- Remove obsolete sections from setup.cfg
- Use newer openstackdocstheme version
- Remove install_command from tox.ini, the default is fine
- Cleanup tox.ini, the repo is now python3 only
Change-Id: Ia24cc501831202cb4d710a9a5e9b2fdb8bef070d