gh-98552: flush std streams in the multiprocessing forkserver before fork

[gpshead]

A minor bug discovered by multiple authors working on a fix for the original bug in the linked issue.

• Issue: multiprocessing forkserver does not flush output before fork (was: preloading '__main__' with forkserver has been broken for a long time) #98552

[miss-islington-app]

Thanks @gpshead for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-141851 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-141852 is a backport of this pull request to the 3.13 branch.

[gpshead]

looks like this was already done for #135335, just earlier in the code from where I had been looking. reverting.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.13 (tier-1) has failed when building commit e2d320b.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1441/builds/1592) and take a look at the build logs.
4. Check if the failure is related to this commit (e2d320b) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1441/builds/1592

Failed tests:

• test_os

Failed subtests:

• test_timerfd_ns_poll - test.test_os.TimerfdTests.test_timerfd_ns_poll

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/root/buildarea/3.13.angelico-debian-amd64/build/Lib/test/test_os.py", line 4342, in test_timerfd_ns_poll
    self.check_timerfd_poll(True)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.13.angelico-debian-amd64/build/Lib/test/test_os.py", line 4327, in check_timerfd_poll
    self.assertEqual(self.read_count_signaled(fd), 1)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 4 != 1


Traceback (most recent call last):
  File "/root/buildarea/3.13.angelico-debian-amd64/build/Lib/test/test_os.py", line 4342, in test_timerfd_ns_poll
    self.check_timerfd_poll(True)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/root/buildarea/3.13.angelico-debian-amd64/build/Lib/test/test_os.py", line 4327, in check_timerfd_poll
    self.assertEqual(self.read_count_signaled(fd), 1)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 2 != 1