BLD: pyproject.toml for Py38

[TomAugspurger]

Closes #34666

[TomAugspurger]

Merging in a few hours if there's no objection.

[jreback]

lets merge now!

[simonjayhawkins]

@meeseeksdev backport to 1.0.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 1.0.x
$ git pull

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

$ git cherry-pick -m1 08febd2b9f77fd8f943de92939788e8bc53ee2d2

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

$ git commit -am 'Backport PR #34667: BLD: pyproject.toml for Py38'

4. Push to a named branch :

git push YOURFORK 1.0.x:auto-backport-of-pr-34667-on-1.0.x

5. Create a PR against branch 1.0.x, I would have named this PR:

"Backport PR #34667 on branch 1.0.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.