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Right, thread.allocate_lock() does allocate an Event on 
Windows.  On other platforms it allocates other kinds of 
limited resource (one way or another, it has to grab some 
kind of locking object from the operating system, and the 
supply of those is typically finite).

Looking it over, I agree Thread.__block won't be used by a 
dummy thread.  I also <wink> note that the use of "assert 
False" in _DummyThread.join() is plain bizarre.  It's a user 
error to try to join a dummy thread, not an internal invariant 
we believe *cannot* occur.  Guido misused (IMO) `assert` in 
several places in this module, but its use in join() is way over 
the edge.

Anyway, ya, I think it would be good to change this.  I don't 
really understand the point to nuking Thread.__stderr -- 
typically, sys.stderr is a single object over the life of a 
program, and doesn't go away until the program ends.  That 
is, I doubt the reference is keeping anything alive that would 
have gone away otherwise.