message: Fix JSON serialization error in TensorFlowOpLayer in Python 3.
diff: @@ -2387,6 +2387,8 @@ class TensorFlowOpLayer(Layer):
  dtype=None):
  super(TensorFlowOpLayer, self).__init__(
  name=_TF_OP_LAYER_NAME_PREFIX + name, trainable=trainable, dtype=dtype)
  + if not isinstance(node_def, bytes):
  + node_def = node_def.encode('utf-8')
  self.node_def = node_def_pb2.NodeDef.FromString(node_def)
  self.constants = constants or {}
  # Layer uses original op unless it is called on new inputs.
  @@ -2446,7 +2448,7 @@ class TensorFlowOpLayer(Layer):
  def get_config(self):
  config = super(TensorFlowOpLayer, self).get_config()
  config.update({
  - 'node_def': self.node_def.SerializeToString(),
  + 'node_def': self.node_def.SerializeToString().decode('utf-8'),
  'constants': {
  i: backend.get_value(c) for i, c in self.constants.items()
  }
  