I want to be able to create a subtype of weakref.

Motivation: I use a trick to non-intrusively keep one 
Python object (ward) alive as long as another one 
(custodian) is: I build a weak reference to the 
custodian whose kill function object holds a reference 
to the ward. I "leak" the weakref, but the function 
decrements its refcount so it will eventually die. 
This scheme costs an extra allocation for the function 
object, and because there is a function object at all, 
there's no opportunity to re-use the weakref (please 
document this part of the re-use behavior, BTW!)

I also want the re-use algorithm to check for object 
and type equality so that I can avoid creating 
multiple such references.