gh-139462: Make the ProcessPoolExecutor BrokenProcessPool exception report which child process terminated

[jberg5]

See #139462 for more context.

Short summary: prior to this change, if a child process segfaulted when running in a concurrent.futures.ProcessPoolExecutor, the user would get a BrokenProcessPool exception with no information about which child process terminated or why.

In order to improve the debugging experience, this change attempts to report which child process terminated and with what exit code. For instance, if I have a worker process that segfaults, I'll now see something like:

Lib.concurrent.futures.process._RemoteTraceback: Process 48939 terminated abruptly with exit code -11

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
...
Lib.concurrent.futures.process.BrokenProcessPool: A process in the process pool was terminated abruptly while the future was running or pending.

• Issue: Hard to debug subprocess crashes when using concurrent.futures.ProcessPoolExecutor #139462

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[python-cla-bot]

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[ZeroIntensity]

Per the bot, please add a news entry, and please also update the "What's New in Python 3.15" document.

[bedevere-app]

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[ZeroIntensity]

Just down to docs nitpicks for me. The rest looks good.

[jberg5]

There was a fun issue where the precommit merge conflict checker thought that a perfectly legitimate string was a problem:

check for merge conflicts................................................Failed
Doc/whatsnew/3.15.rst:680: Merge conflict string '=======' found

I removed one of the = and it was happy again :) (and then added it back after)

[jberg5]

Thanks @ZeroIntensity, appreciate all the feedback!

[ZeroIntensity]

Thanks, this looks good to me. I'll wait for @gpshead as the multiprocessing expert to decide on the weird triple-quotes in the message.

[ZeroIntensity]

Hmm, something seems to be causing the tests to hang.

[jberg5]

Hi @ZeroIntensity and @gpshead! Tests are passing, let me know if there are any other changes you'd like me to make, or if this is good to go.

[jberg5]

@AA-Turner @ZeroIntensity @gpshead thanks for all the feedback thus far! Let me know if there's anything else you'd like me to change here.