rather than in docstrings.  Rewrote so that _active_limbo_lock is released
no matter what happens (it could have been left locked if _sys got None'd
out).  Use "in" in preference to has_key() for dict lookup.  Don't bother
looking for 'dummy_threading' in sys.modules unless KeyError is raised.
Since the heart of the method is the del, do that in only one place.