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
- Update classifiers
- Remove install_command from tox.ini, the default is fine
- Stop py35 testing, OpenStack has py36 as minimal version
Change-Id: Iee8ffab15e11afe417f7369d907491b62ba063fb