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
- Update classifiers
- Update requirements, no need for python_version anymore
- Switch to building docs with sphinx-build, use apidoc here and fix
  problems found.

Change-Id: I64c07c9e1621518240bfb60efa7f0990b0e7e727