Make a few cleanups:
- Remove python 2.7 stanza from setup.py
- Add requires on python >= 3.6 to setup.cfg so that pypi and pip
know about the requirement
- Remove section from setup.cfg: Wheel is not needed for python
3 only repo
- Update requirements, no need for python_version anymore
Change-Id: Ib2221fc13cd53c93150e838418e7e110e087d1b3