gh-128639: Don't assume one thread in subinterpreter finalization with fixed daemon thread support

[ZeroIntensity]

This is a reapplication of #128640, with daemon thread hanging fixed.

The key part that I was missing was this:

for (PyThreadState*p=list; p!=NULL; p=p->next) {
_PyThreadState_SetShuttingDown(p);
}

My fix predated these lines, so it didn't make it in originally.

The reason this only failed on the iOS buildbots was because they're single-process. Out of random choice, I picked 100 seconds as the sleep time, but since the process would exit before that 100 seconds was over on all the other buildbots, there wasn't any problem. On iOS, the test suite continued to run in the same process after the 100 seconds were up, so the daemon threads would start trying to run again under a deallocated interpreter, causing all sorts of weird crashes.

• Issue: Subinterpreters don't properly clean up threads #128639

[bedevere-bot]

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[ZeroIntensity]

!buildbot refleak

[bedevere-bot]

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[ZeroIntensity]

@ericsnowcurrently Yay, buildbots passed on this one!

[ericsnowcurrently]

Like with gh-128640, mostly LGTM. I've left a few minor comments.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @ZeroIntensity for commit 62bd653 🤖

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If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[ZeroIntensity]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@ericsnowcurrently: please review the changes made to this pull request.

[ericsnowcurrently]

LGTM

[miss-islington-app]

Thanks @ZeroIntensity for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-app]

GH-139050 is a backport of this pull request to the 3.14 branch.