gh-140648: Make asyncio REPL respect the -I flag (isolated mode)

[johnslavik]

• Issue: The asyncio REPL reaches for PYTHONSTARTUP in isolated mode #140648

[miss-islington-app]

Thanks @johnslavik for the PR, and @kumaraditya303 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-143383 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-143384 is a backport of this pull request to the 3.13 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot ARM64 MacOS M1 Refleaks NoGIL 3.13 (tier-2) has failed when building commit dced1a7.

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/1396/builds/1918) and take a look at the build logs.
4. Check if the failure is related to this commit (dced1a7) 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/1396/builds/1918

Failed tests:

• test.test_multiprocessing_spawn.test_processes

Failed subtests:

• test_repr_rlock - test.test_multiprocessing_spawn.test_processes.WithProcessesTestLock.test_repr_rlock

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

==

Click to see traceback logs

Traceback (most recent call last):
File "/Users/buildbot/buildarea/3.13.itamaro-macos-arm64-aws.macos-with-brew.refleak.nogil/build/Lib/test/_test_multiprocessing.py", line 1493, in test_repr_rlockself.assertEqual('<RLock(SomeOtherThread, nonzero)>', repr(lock))
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^AssertionError: '<RLock(SomeOtherThread, nonzero)>' != '<RLock(None, 0)>'
- <RLock(SomeOtherThread, nonzero)>
+ <RLock(None, 0)>