Any reason you don't want to call set_next from set_get?

I would say

static PyObject *
set_get(PySetObject *so)
{
	register Py_ssize_t pos = 0;
	register setentry *entry;
        if (set_next(so, &pos, &entry)) {
              Py_INCREF(entry->key);
              return entry->key;
        }
        /* set appropriate error */
        return NULL;
}

BTW, what your patch is supposed to do on set().get()?
}