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Starting with commit 468e5fec (bpo-36402: Fix threading._shutdown() race condition (GH-13948)) the following trivial test case leaks one reference and one memory block.
class MiscTestCase(unittest.TestCase):
 def test_without_join(self):
 # Test that a thread without join does not leak references.
 # Use a debug build and run "python -m test -R: test_threading"
 threading.Thread().start()
Attached is a patch, that adds this test case to Lib/test/test_threading.py. After you apply this patch "python -m test -R: test_threading" leaks one (additional) reference. This leak is also present in Python 3.7.4 and 3.8.
I'm not sure, if it correct not to join a thread, but it did work flawlessly and didn't leak in previous releases.
I didn't analyse the root cause yet.