Added test for uintptr_t, the C9X acceptable way to spell "type to

Added test for uintptr_t, the C9X acceptable way to spell "type to
which I can cast void* to and back again without losing information".
In pyport.h, we typedef Py_uintptr_t to mean this thing, which if the
platform supports, will be uintptr_t (otherwise, other accomodations
are made).