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 doc/source/conf.py to remove now obsolete content.
- Move basepython to testenv, to use it everywhere in tox.ini.
Change-Id: I1f06b471e1a1ce1570862b281bdff82d27c6d381