A new behavior was introduced in r59106 in python trunk, which look
suspicious to me. Basically, every time a class defines a comparison
operator, it's __hash__ method is defined to None. The simple following
example shows it:

>>> class A(object):
...     def __init__(self, b):
...             self.b = b
...     def __cmp__(self, other):
...             if not isinstance(other, A):
...                     return -1
...             return cmp(self.b, other.b)
...
>>> hash(A(2))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable


The problematic change is here:
http://svn.python.org/view/python/trunk/Objects/typeobject.c?rev=59106&r1=58032&r2=59106

And mainly the overrides_hash call in inherit_slots.

FWIW, I've encounter the problem because zope.interface is now usable on
trunk.