Consider changes needed/possible when Python 3.6 reaches end of life

[neiljp]

This is the next iteration of #567, and is mainly for tracking. See that issue for potentially useful tools.

3.6 is due to end 23 Dec 2021 according to https://endoflife.date/python or more officially https://devguide.python.org/#status-of-python-branches

[neiljp]

Some updates in our requirements/dependencies are slightly blocked in updating further based on this, so once this transition is done (or part of it) we can check dependencies for that.

Examples:

• pytest 7
• pytest-mock 3.7