Attached patch allows classes to explicitly block inheritance of
object.__hash__ by setting the tp_hash slot to Py_None or the __hash__
attribute to None.

This is similar to the approach used in Py3k, but allows __hash__ to be
inherited by default, with classes explicitly disabling it as
appropriate. I'd also suggest forward porting this approach to Py3k as
well, and then possibly look at replacing a copied tp_hash slot with
Py_None when the inherited tp_hash is object.tp_hash, but tp_cmp or
tp_richcompare are different.