gh-108696: revert bypassing import cache in test_import helper

[carljm]

Per suggestion from @FFY00 on #104186, let's revert this change to get the refleak buildbots green again.

Verified that with this diff, ./python -m test -R: -v test_import no longer leaks references.

It seems that if we want to bypass this cache, it will require a bit more work to ensure we always dispose of the freshly-imported modules.

• Issue: test_import leaks references #108696

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 RHEL8 LTO 3.x has failed when building commit 157b89e.

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/all/#builders/338/builds/4926) and take a look at the build logs.
4. Check if the failure is related to this commit (157b89e) 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/all/#builders/338/builds/4926

Failed tests:

• test.test_concurrent_futures.test_shutdown

Failed subtests:

• test_interpreter_shutdown - test.test_concurrent_futures.test_shutdown.ProcessPoolSpawnProcessPoolShutdownTest.test_interpreter_shutdown

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

== Tests result: FAILURE then FAILURE ==

447 tests OK.

10 slowest tests:

• test_gdb: 3 min 25 sec
• test.test_concurrent_futures.test_wait: 1 min 9 sec
• test_signal: 1 min 3 sec
• test.test_multiprocessing_spawn.test_processes: 1 min 1 sec
• test_socket: 50.9 sec
• test_io: 40.9 sec
• test.test_multiprocessing_forkserver.test_processes: 40.9 sec
• test_math: 33.1 sec
• test_subprocess: 32.9 sec
• test_imaplib: 32.0 sec

1 test failed:
test.test_concurrent_futures.test_shutdown

14 tests skipped:
test.test_asyncio.test_windows_events
test.test_asyncio.test_windows_utils test_devpoll test_ioctl
test_kqueue test_launcher test_startfile test_tkinter test_ttk
test_winconsoleio test_winreg test_winsound test_wmi
test_zipfile64

1 re-run test:
test.test_concurrent_futures.test_shutdown

Total duration: 3 min 32 sec

Click to see traceback logs

Traceback (most recent call last):
File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/test/test_concurrent_futures/test_shutdown.py", line 49, in test_interpreter_shutdownself.assertFalse(err)
AssertionError: b'Exception in thread Thread-1:\nTraceback (most recent call last):\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 1059, in _bootstrap_inner\n self.run()\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/concurrent/futures/process.py", line 339, in run\n self.add_call_item_to_queue()\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/concurrent/futures/process.py", line 394, in add_call_item_to_queue\n self.call_queue.put(_CallItem(work_id,\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/multiprocessing/queues.py", line 94, in put\n self._start_thread()\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/multiprocessing/queues.py", line 177, in _start_thread\n self._thread.start()\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/threading.py", line 978, in start\n _start_new_thread(self._bootstrap, ())\nRuntimeError: can\'t create new thread at interpreter shutdown\nTraceback (most recent call last):\n File "<string>", line 1, in <module>\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/multiprocessing/spawn.py", line 122, in spawn_main\n exitcode = _main(fd, parent_sentinel)\n ^^^^^^^^^^^^^^^^^^^^^^^^^^\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/multiprocessing/spawn.py", line 132, in _main\n self = reduction.pickle.load(from_parent)\n ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n File "/home/buildbot/buildarea/3.x.cstratak-RHEL8-aarch64.lto/build/Lib/multiprocessing/synchronize.py", line 115, in __setstate__\n self._semlock = _multiprocessing.SemLock._rebuild(*state)\n ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\nFileNotFoundError: [Errno 2] No such file or directory\n' is not false