py3: add one new passing test found by buildbot Differential Revision: https://phab.mercurial-scm.org/D6220

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.