an ExtensionClass type that get into this code can raise RuntimeWarning
under Python 2.3, because the comparison result may be outside the
set {-1, 0, 1}.  Worse, on a box where sizeof(ptrdiff_t) > sizeof(int)
(all 64-bit platforms), comparison results across a set of objects
could be inconsistent because the implicit cast of pointer subtraction
to int loses the information-carrying sign bit.