The memory layout of PyType object's changes in Py3k from the
*compiler's* point of view. This means PyObject_HEAD_INIT can no longer
be used to initialise PyVarObject type definitions.

However, the documentation doesn't point this out (or document
PyVarObject_HEAD_INIT at all), and the compiler warnings currently
generated are not clear. Suggestion is to remove PyObject_HEAD_INIT from
Py3k entirely so this becomes a compile error instead of a warning, and
then better document the situation so extension authors know how to
correctly initialise the affected C structs.

See mailing list thread at:
http://mail.python.org/pipermail/python-3000/2008-November/015241.html