Clarify comment

Author: Yury Selivanov

tests/test_future/test_bytes.py

@@ -588,6 +588,9 @@ def test_mod(self):
 
         The bytes mod code is in _PyBytes_Format() in bytesobject.c in Py3.5.
         """
+
+        # XXX Add support for %b!
+        #
         # b = bytes(b'hello, %b!')
         # orig = b
         # b = b % b'world'