bpo-31234: test_threading: fix ref cycle (#3150)

test_bare_raise_in_brand_new_thread() now explicitly breaks a
reference cycle to not leak a dangling thread.

Author: vstinner

Lib/test/test_threading.py

@@ -1069,6 +1069,8 @@ def run(self):
         thread.join()
         self.assertIsNotNone(thread.exc)
         self.assertIsInstance(thread.exc, RuntimeError)
+        # explicitly break the reference cycle to not leak a dangling thread
+        thread.exc = None
 
 class TimerTests(BaseTestCase):