Ensure tox uses oldest supported Python for coverage

[rmartin16]

nedbat/coveragepy#1572 (comment)

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Looks good - but one question about the exactly what is being added/changed here.

[freakboy3742]

Clarifying the interpretation here - The behavior in CI definitely seems to be using the py38 environment; but does this definition mean it will prefer the py38 environment, but if you don't have one, it will use py39, and so on? The tox documentation isn't clear about this (to me, anyway), and it's difficult to test.

[rmartin16]

Yeah....everything about tox just feels a little cryptic. Nonetheless, tox supports a whole "discovery" algorithm for finding versions of Python; depending on PATH is mostly working for me, though.

This setting means use the first available version of Python in this list.

The easiest way I found to test it is to make sure Python 3.8 or Python 3.9 is in PATH and start tox from a virtual env using a different Python version. You should see tox create an environment using 3.8...sometimes adding -v to tox makes things clearer....or even more confusing...

Finally, CI is was using Python 3.8 even before this because that's what's installed for that CI job and the default choice for which python.

[freakboy3742]

Ok - that's enough of a clarification for me; it's working in my (limited) testing, and in CI, so I'm happy to run with that.