Issue1200

I know that Java weak reference are implemented differently, e.g. all
objects are inherently weak-referencable and __weakref__ slot is
meaningless.  Still, Jython could add special handling for the slot to
simplify things for code that blindly (not checking exact
implementation) uses it.

E.g.:

class X (object):
    __slots__ = ('__weakref__')

x = X ()
x.__weakref__ = 1

This code fails in CPython but works fine in Jython.  However, it wastes
one slot, making each object unnecessarily larger (I admit I don't
really know how Jython works, but that's what I guess it results in). 
Also, it seems Jython is capable of not creating duplicate weakref object:

>>> a = object ()
>>> b = weakref.ref (a)
>>> c = weakref.ref (a)
>>> b, c, b is c
(<weakref at 1; to 'object' at 2>, <weakref at 1; to 'object' at 2>, True)

so x.__weakref__ (descriptor getter) could be implemented as in CPython.

I'm not proposing to artificially limit set of weak-referencable types
--- I'm just asking to make __weakref__ slot "just work as intended"
without caring whether it is CPython or Jython.

I've been intending to do this for 2.5, thanks for the reminder

fixed in r6198, thanks