bpo-31234: Join timers in test_threading (#3598)

Call the .join() method of threading.Timer timers to prevent the
"threading_cleanup() failed to cleanup 1 threads" warning.

Lib/test/test_threading.py

@@ -1099,6 +1099,8 @@ class TimerTests(BaseTestCase):
         self.callback_event.wait()
         self.assertEqual(len(self.callback_args), 2)
         self.assertEqual(self.callback_args, [((), {}), ((), {})])
+        timer1.join()
+        timer2.join()
 
     def _callback_spy(self, *args, **kwargs):
         self.callback_args.append((args[:], kwargs.copy()))