This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove obsolete sections from setup.cfg
- Cleanup doc/source/conf.py to remove now obsolete content.
- Use newer openstackdocstheme and Sphinx versions
- Remove install_command from tox.ini, the default is fine
Change-Id: I4980bac7a558a291963270de592078f1a3af1111