This issue is similar to issue1549. Note that only new-style classes are
concerned.

I think the change was intentional. Guido, do you confirm?

However there should be some documentation about it, a NEWS entry or an
item in the "porting to 2.6" section.

The initial modification appeared in the py3k branch (r51454):
"""
Change the way __hash__ is inherited; when __eq__ or __cmp__ is
overridden but __hash__ is not, set __hash__ explicitly to None (and
tp_hash to NULL).
"""