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
- Remove html_last_updated_fmt from source/conf.py, this is not
needed anymore.
Change-Id: If9708106263493396693771e9bd5265e26de460c