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Python 3.0 shouldn't have any problem unpickling old-style classes.
However, they will be unpickled as new-style classes. Therefore, there
might be a few corner-cases that might cause some problems. For example,
if an old-style class contains a __slots__:
Python 2.5.1 (r251:54863, Mar 7 2008, 04:10:12) 
>>> import pickle
>>> class A:
... __slots__ = []
... 
>>> pickle.dumps(A())
'(i__main__\nA\np0\n(dp1\nb.'
Python 3.0a3+ (py3k:62050M, Mar 30 2008, 17:29:33) 
>>> class A:
... __slots__ = []
... 
>>> pickle.loads(b'(i__main__\nA\np0\n(dp1\nb.')
Traceback (most recent call last):
 ...
TypeError: __class__ assignment: '_EmptyClass' deallocator differs from 'A'