message: Replaced explicit __metaclass__ assignment with
  `__metaclass__ `only works for Python 2. In Python 3 it has no effect
  since the metaclass syntax is
  class A(meta=abc.ABCMeta):
  ...
  `@six.add_metaclass` covers both variants.
diff: @@ -22,6 +22,8 @@ import socket
  import ssl
  import struct
  +import six
  +
  from tensorflow.contrib.ignite.python.ops import gen_dataset_ops
  from tensorflow.contrib.ignite.python.ops import ignite_op_loader # pylint: disable=unused-import
  from tensorflow.python.data.ops import dataset_ops
  @@ -30,6 +32,7 @@ from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor_shape
  +@six.add_metaclass(abc.ABCMeta)
  class Readable(object):
  """Readable abstract class that exposes methods to do reading-related
  