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
  * Some other sections are obsolete
- Update classifiers
- Update requirements, no need for python_version anymore
- Remove html_last_updated_fmt from conf.py, this is not needed with
  openstackdocstheme anymore; update openstackdocstheme requirement

Change-Id: I5389ba08c985e023643c07adf3135809c291146f

Now that we are py36 or later, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: I9582d623727e5853637812083033a348b71551ce
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

Per the Ussuri cycle goal, this drops py27 testing and support.

https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I0c525eb48270494e8171042e56e14867c53e5af3
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

Change-Id: I8213aea6bb576ff312c238c8f2ea85fafe318d97

Add file to the reno documentation build to show release notes for
stable/train.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/train.

Change-Id: Ibb13de8b19df12cc01776b1b015a836c7cff07dc
Sem-Ver: feature

During the train cycle, a new static URL scheme was introduced for
upper-constraints URLs that allows us to switch locations based on
configuration and avoid timing windows with the requirements repo
branching. This updates upper-constraints links in the tox.ini file to
point to the newer more prefered location.

Change-Id: I8efbb24483d2a63a8abfa50e328fefb7ffd92e5f
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>