I have a few spare cycles to take another stab at this issue. I can say with some certainty that the failure in test__xxsubinterpreters.DestroyTests does not occur on the latest commit to master (3.9):

```
$ ./python -m test test__xxsubinterpreters --match DestroyTests -j200 -F -v

OK
0:09:28 load avg: 80.87 [2188] test__xxsubinterpreters passed
test_all (test.test__xxsubinterpreters.DestroyTests) ... ok
test_already_destroyed (test.test__xxsubinterpreters.DestroyTests) ... ok
test_bad_id (test.test__xxsubinterpreters.DestroyTests) ... ok
test_does_not_exist (test.test__xxsubinterpreters.DestroyTests) ... ok
test_from_current (test.test__xxsubinterpreters.DestroyTests) ... ok
test_from_other_thread (test.test__xxsubinterpreters.DestroyTests) ... ok
test_from_sibling (test.test__xxsubinterpreters.DestroyTests) ... ok
test_main (test.test__xxsubinterpreters.DestroyTests) ... ok
test_one (test.test__xxsubinterpreters.DestroyTests) ... ok
test_still_running (test.test__xxsubinterpreters.DestroyTests) ... ok

== Tests result: INTERRUPTED ==
Test suite interrupted by signal SIGINT.

2188 tests OK.
```

So at this point it seems to be a matter of looking through the diff between 3.8 vs 3.9 for any relevant code paths, and attempting to determine what change resolved the failure for 3.9. It might also be useful to determine if it occurred when the tests were first added in 3.8 or if some other commit introduced a subtle regression.