[3.14] gh-135427: Fix DeprecationWarning for os.fork when run in threads with -Werror (GH-136796)

[gpshead]

cherry-pick fd8f42d for a backport to 3.14

Don't ignore errors raised by PyErr_WarnFormat in warn_about_fork_with_threads Instead, ignore the warnings in all test code that forks. (That's a lot of functions.)

In test_support, make ignore_warnings a context manager (as well as decorator), and add a message argument to it.
Also add a ignore_fork_in_thread_deprecation_warnings helper for the deadlock-in-fork warning.

• Issue: -Werror doesn't error out on DeprecationWarning for os.fork() #135427

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[encukou]

Should we backport this?

Warnings changes in patch releases make it harder to update, and from comments like #135427 (comment) it sounds like it's important to fix this now.

But, if you think it should get a backport, let's do it :)

[gpshead]

I don't think this one is actually urgent to backport. I'll close this, the PR is ready and can be resurrected if deemed important.

So -Werror doesn't turn it into an error in 3.14 but we've fixed that in 3.15? good enough. This is an odd warning - it'll probably never become an error because our detection can only ever be best effort (it cannot be guaranteed to detect if other threads exist), it's more of the best way to surface an application design flaw to users.