Add support for latest Python versions

[at88mph]

Python 3.8 will be released soon, Python 3.7 was never supported, and Python 2.7 support will disappear as of January 2020.

A sensible list of supported versions are on the Docker Hub:
https://hub.docker.com/_/python

I think the list of supported versions there is a sensible list.

[andamian]

3.7 it's on my list of things to do - thanks for opening the issue.

@ijiraq might want to comment on a timetable for weaning our users off P2

[andamian]

Overcome by events. We are at 3.9 currently but still supporting 2.7...