PyString_AsString is permissive and accepts unicode strings.

Replace it with PyUnicode_AsString when the argument is known to be a str.

Objects/setobject.c

@@ -2303,7 +2303,7 @@ test_c_api(PySetObject *so)
 	/* Exercise direct iteration */
 	i = 0, count = 0;
 	while (_PySet_Next((PyObject *)dup, &i, &x)) {
-		s = PyString_AsString(x);
+		s = PyUnicode_AsString(x);
 		assert(s && (s[0] == 'a' || s[0] == 'b' || s[0] == 'c'));
 		count++;
 	}