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Marked accepted.  Looks good!

Suggest

return a->ob_size == b->ob_size &&
       *a->ob_sval == *b->ob_sval &&
       memcmp(a->ob_sval, b->ob_sval, a->ob_size) == 0;

for the tail of the _PyString_Eq body as compilers should 
have an easier time of turning that into the best code for 
the platform (especially the weaker compilers do better 
optimizing expressions than across branches).  Plus it 
improves clarity, at least for me.

Unsure why the

case Py_EQ: c = c == 0; break; /* not needed here */

case is there:  if it's truly unreacable (and I agree it 
isn't), better to assert-fail if it gets there.