PPC64 AIX 3.x:
https://buildbot.python.org/all/#/builders/10/builds/2224

0:12:47 [160/420/1] test_threading failed (env changed)
...
Ran 158 tests in 12.023s

OK (skipped=1)
Warning -- files was modified by test_threading
  Before: []
  After:  ['core']


I don't know if it's a regression or if it occurred in the past.

Michael: Can you please try to reproduce the issue? Any idea if it's new or not?

You can try to identify which test creates a core dump using:

$ ./python -m test.bisect_cmd --fail-env-changed test_thread -v

The command is supposed to find which test creates the "core" file.