Perfect---thank you!  Applied to trunk in r76963.  I tweaked the main 
comment and docstring, wrapped a long line, and replaced two instances of 
'*overflow = Py_SIZE(v) > 0 ? 1 : -1' with simply '*overflow = sign'.  I 
also expanded the test a bit to check for 1 and -1 *overflow values, 
rather than just checking for nonzero *overflow.

I'll propagate your tests to py3k.