Verify support for Python 3.9 (once it is more established)

[sadielbartholomew]

Python 3.9 was released a few days ago (see e.g. here). Though it is clearly early days for 3.9, to keep up-to-date with supported Python versions, we should soon check that everything works as it should on 3.9 as with 3.6 - 3.8 (I've just dropped 3.5 support as it is end-of-life as of last month), verify this via the Actions workflow, and document whether 3.9 is good to use.

To that aim, I have already tried adding in 3.9 to the Python version job matrix on our Actions CI workflow, since it is confirmed as being available for GH Actions (see here). Unfortunately it is failing on the miniconda step (see e.g. this 3.9 job run output), so it seems that one or both of the following are true (as also suggested here):

• an issue in installing 3.9 with conda, at least via the default channel;
• or (much more likely) dependencies don't yet support 3.9, so he environment can't be set up to support 3.9 regardless of our code.

There is a nice little article here discussing the switch to 3.9 and (see the final section) advising when it is sensible to consider 3.9 support. Taking that advice which seems very sensible, we should review and verify support for 3.9 in around mid-December '20.

For now, I will remove 3.9 jobs from the Actions workflow as they will fail and cancel all other jobs.

[davidhassell]

Reviewing mid December 2020 sounds good to me, too. Thanks.

[sadielbartholomew]

This one was resolved long ago, so closing belatedly...