I think the right way to handle this is to modify the test:

if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
    !PyObject_TypeCheck(args, &PyBaseString_Type))

to also exclude PyStructSequence's, but since they're all distinct types I don't see how to do that. I don't really think listing all of the PyStructSequence types would be acceptable.

This is the test that's trying to figure out if %-formatting should use mapping or tuple expansion.