SF #515023. Make _DummyThread.join() signature match base class (Thread)

Lib/threading.py

@@ -572,7 +572,7 @@ class _DummyThread(Thread):
     def _set_daemon(self):
         return 1
 
-    def join(self):
+    def join(self, timeout=None):
         assert 0, "cannot join a dummy thread"