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Hm, this is serious.

This can happen whenever a built-in type
isn't initialized by PyType_Ready() yet,
and for many that is pretty common.
(The call to int.__mro__ goes through
type_getattro which calls PyType_Ready().)

I'm not sure which refactoring patch
you are referring to, and I'm actually
not at all sure that this can't happen
in 2.3 (despite the fact that this
particular example doesn't core dump there).

I'll investigate more...