[Python-3000] Draft PEP: Dropping PyObject_HEAD

["Martin v. Löwis"]

> - add the new access macros to 2.6 (defined appropriately)
> - in 3.0, instead of removing PyObject_HEAD and company, redefine them
> appropriately, .e.g, #define PyObject_HEAD PyObject obtype;

Thanks, I added both of them into PEP 3123 (now retitled to
indicate that PyObject_HEAD is not dropped).

Regards,
Martin