Make a few cleanups:
- Remove obsolete sections from setup.cfg
- Switch to using sphinx-build; fix warnings found:
convert unreferenced footnote into normal link
- Cleanup doc/source/conf.py to remove now obsolete content.
- Use newer openstackdocstheme version, update some other versions as
needed
- Remove install_command from tox.ini, the default is fine; cleanup
for python3 only
Change-Id: If18af73f3886ad32622801193f7f7ae53458b50c