gh-87063: Add closed attribute to multiprocessing.Process

[rruuaanng]

• Issue: Expose a way to determine if a process has been closed or not #87063

for example:

import multiprocessing


def f():
    print('hello')
    print('world')

if __name__ == "__main__":
    p = multiprocessing.Process(target=f)
    p.start()
    p.join()

    # In a more context, it's closed
    p.close()
    
    # Currnet context
    # It's thrown a ValueError
    if p.exitcode == 0:
        pass

    # Changed
    if not p.is_closed():
        print(p.exitcode)

[rruuaanng]

It seems that this NEWS error can be ignored.

[picnixz]

More generally, required CI checks must never be ignored except for NEWS missing / issue missing since normal users cannot apply the skip news/issue labels themselves.

[erlend-aasland]

PR author did not listen to feedback. Requesting changes to make this point clearer.

[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.

[rruuaanng]

Sorry for my misunderstanding. I thought it was to change NEWS to what's NEWS, but it was actually to add to it. I'm sorry for that. I've made change :)

[rruuaanng]

Thanks to Bénédikt for taking responsibility for this PR. And, thanks to erlend for pointing out my mistake.👍

[picnixz]

A final nitpick. Concerning the naming of the attribute, I suggested a property + closed but if @gpshead feels that it should be a method + is_closed(), then I'm sorry in advance :) (just recalling that my rationale was that multiprocessing.connection.Connection uses the closed property but I don't know if properties are preferred over true methods when we are just returning a private attribute).

[rruuaanng]

I've made change. Thank for your review!