Figure out a plan for EOLing python2 support.

Document and publish.

I figure we have at least 3 more years before we stop supporting python2.

New features if they can be supported with both python2 and 3 easily
should be. But I have no objection to adding features that only work on python3
and return not implemented if used on python2.

Will need to figure out how to mark these python3 only features in documentation.

GitHub CI is removing support for python 2 from their setup.

So this might drive a nail in the coffin earlier that I had hoped.

https://github.com/actions/setup-python/issues/672

I still have python 2.7 on travis-ci but I keep running out of (free for now) credits there 
so... CI isn't reliable as it doesn't notify me when I am out of credits and thus I don't kno 
the difference between not running and a failure on a run.

Added warning to plan on changing to Python 3 (or not using python 2) to installation and
upgrading docs.

changeset:   7438:116ea5ce06ab

Still need a plan and an EOL date.