[3.10] gh-91676 gh-91260 unittest.IsolatedAsyncioTestCase no longer leaks its executor

[gpshead]

For things like test_asyncio.test_thread this was causing frequent
"environment modified by test" errors as the executor threads had not
always stopped running after the test was over.

issues #91676 and #91260 alternate fix to #91667.

This only happened in 3.10 and 3.9 after #32086 as the main 3.11 branch has a completely rewritten unittest.IsolatedAsyncioTestCase that uses asyncio APIs itself that do not appear to leak the executor.

[gpshead]

@asvetlov is this unittest.IsolatedAsyncioTestCase change to make in stable releases or should #91667 proceed to simply revert your test_asyncio.test_threads.py PR in the 3.10 and 3.9 branches?

I believe it to be what people want this class to do. But https://docs.python.org/3.11/library/unittest.html#unittest.IsolatedAsyncioTestCase.run says nothing more than "sets up a new event loop to run the test ... at the end tasks in the event loop are cancelled" without explicitly stating what should be obvious that upon exit this new private event loop should go away.

So I believe this unittest bugfix is desirable. There appears no nice way to inject this shutdown_default_executor() call as a workaround for anyone already using this class given the way the code in 3.10 and earlier is structured.

[miss-islington]

Thanks @gpshead for the PR, and @pablogsal for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-91692 is a backport of this pull request to the 3.9 branch.