cext: cast s# arguments of Py_BuildValue() to Py_ssize_t
The doc doesn't state that "s#" of Py_BuildValue() is controlled by
PY_SSIZE_T_CLEAN (unlike the one for PyArg_ParseTuple()), but actually
it's switched to Py_ssize_t.
https://docs.python.org/2/c-api/arg.html#c.Py_BuildValue
https://github.com/python/cpython/blob/2.7/Python/modsupport.c#L432
Follow up for
b01bbb8ff1f2 and
896b19d12c08.
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