trampolining going on with the tp_new descriptor, where the inherited
PyType_GenericNew was overwritten with the much slower slot_tp_new
which would end up calling tp_new_wrapper which would eventually call
PyType_GenericNew.  Add a special case for this to update_one_slot().

XXX Hope there isn't a loophole in this.  I'll buy the first person to
point out a bug in the reasoning a beer.

Backport candidate (but I won't do it).