Here's an example of a metaclass with a __cmp__ special:

    exarkun@boson:/tmp$ cat metacmp.py
    class X(type):
        def __cmp__(self, other):
            return -1

    class Y:
        __metaclass__ = X

    print Y < Y
    exarkun@boson:/tmp$ python2.5 metacmp.py
    True
    exarkun@boson:/tmp$ python2.6 metacmp.py
    False
    exarkun@boson:/tmp$

In Python 2.6 and Python 2.7a1, the __cmp__ isn't even invoked, as it
was in Python 2.5.

This breaks the ordering of item types in Axiom.  A brief search reveals
that sympy also ran into the problem, but they may have since worked
around it.

Apparently type has __lt__ and the other rich cmp specials in Python
2.6, but lacked them in Python 2.5.  I suppose these are probably being
given precedence over the overridden __cmp__.

It seems that type was switched to use tp_richcompare in r61529:

Add py3k warnings for object, type, cell and dict comparisons. This
should resolve issue2342 and partly resolve issue2373.

Fixed in r76794.

Note: test_py3kwarn fails, probably since r76794.

Fixed in r76852.