Start testing on current Python versions.

[Carreau]

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[Carreau]

@meeseeksdev backport to 4.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 4.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 39f3bd8ea844bf2367c6b2b7d29535f5d84b6806

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #559: Start testing on current Python versions.'

4. Push to a named branch :

git push YOURFORK 4.x:auto-backport-of-pr-559-on-4.x

5. Create a PR against branch 4.x, I would have named this PR:

"Backport PR #559 on branch 4.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.