bpo-40943: PY_SSIZE_T_CLEAN required for '#' formats (GH-20784)

The PY_SSIZE_T_CLEAN macro must now be defined to use
PyArg_ParseTuple() and Py_BuildValue() "#" formats: "es#", "et#",
"s#", "u#", "y#", "z#", "U#" and "Z#". See the PEP 353.

Update _testcapi.test_buildvalue_issue38913().