This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove obsolete sections from setup.cfg
- Use newer openstackdocstheme and Sphinx versions for Python3
- Cleanup */source/conf.py to remove now obsolete content.
- Remove install_command from tox.ini, the default is fine
- Move python3 to testenv in tox.ini
- Switch to hacking 3, fix problems found.
Change-Id: I26fcab3b288b5ed64c74f7a6725b43036f195d44
7931f9818d