Try to understand why most buildbots suddenly turned to red.

Undo the only change that might have unexpected effects.

To be followed.

Lib/threading.py

@@ -535,7 +535,8 @@ class Thread(_Verbose):
                 # test_threading.test_no_refcycle_through_target when
                 # the exception keeps the target alive past when we
                 # assert that it's dead.
-                self.__exc_clear()
+                # XXX Temporary experiment
+                # self.__exc_clear()
         finally:
             with _active_limbo_lock:
                 self.__stop()