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 obsolete sections from setup.cfg:
  * Wheel is not needed for python 3 only repo
- Update classifiers
- Update requirements, no need for python_version anymore

Change-Id: I705a40cb3a19e16641a2b9de6891ab72fe2b98f0