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Ah,the detail that I was missing was that wr_callback is not
invoked if the weakref list has been cleared.  I.e., calling
_PyWeakref_ClearRef() is enough ensure that the callback is
never implicitly envoked.  That's cleaner than explicitly
clearing the wr_callback pointer and also seems to a
behavior that can be depended on (it's pretty fundamental to
how weakrefs are implemented).

patch3.txt looks like a winner.