Run CI for Python 3.6 on Ubuntu 20.04

[kalikiana]

The now latest Ubuntu no longer ships with 3.6.

This is a partial revert of #38.

[AdamWill]

So I'm not sure I like this, I don't like pinning things on older versions. I think if we really want Python 3.6 testing we might want to do the thing where it's a separate workflow and all the other tests run on ubuntu-latest.

Can you rebase this on #39 and see if stable 3.11 and alpha 3.12 work on 20.04? If those don't work, I'd consider that worse than losing 3.6...

[kalikiana]

No strong opinion. I just wanted to address #38 (comment) and that was the easier option out of the two you suggested 😉
Will give it a try.

[AdamWill]

You need to add 3.6 back to the python-version: list in tox.yml to get 3.6 tests actually running again on github.

[AdamWill]

OK, well, at least all versions work there. The above is just my personal opinion, if everyone else likes this more we can go with it...what does everyone else think? @okurz @mimi1vx @perlpunk

[kalikiana]

OK, well, at least all versions work there. The above is just my personal opinion, if everyone else likes this more we can go with it...

You suggested two ways. I demonstrated both work. Right now it's using a different image for 3.6.

[okurz]

I like this way better because it's explicit that we only need the older ubuntu-20.04 for python 3.6

[AdamWill]

Oh sorry, I didn't notice you'd switched it! That matrix.os thing is neat, nice job. I like it this way, let's see if others have opinions.

[jvanasco]

Thanks for sharing this in /actions/setup-python/issues/543, @AdamWill !

Thank you SO MUCH for devising this solution, @kalikiana ! This is the exact approach I needed to implement in several projects. You've really helped a lot of people today!