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 section from setup.cfg: Wheel is not needed for
python 3 only repo
- Update requirements, no need for python_version anymore
- Remove future import from doc/source/conf.py
Change-Id: Ibf594171cea8f81cc4139b29cbdce54f6d5107a7
b58ebe2b7f