This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove obsolete sections from setup.cfg
- Update requirements, no need for python_version anymore
- Switch to using sphinx-build, fix problems found
- Use newer openstackdocstheme and Sphinx versions
- Cleanup */source/conf.py to remove now obsolete content.
- Remove install_command from tox.ini, the default is fine
Change-Id: I6c593dff382a7aac4d758f802173f5a9f1e66182
b3a0be6eaa