This repo is now testing only with Python 3, so let's make
a few cleanups:
- Remove python 2.7 stanza from setup.py
- Remove obsolete sections from setup.cfg
- Cleanup */source/conf.py to remove now obsolete content.
- remove install_cmd, move constraints into deps,
small cleanup of tox.ini
Change-Id: Ib88cbadb622163d41428b153739679cbba7c336d