message: Update metaclass definition in key_manager.py to be compatible with Python3.
diff: @@ -19,6 +19,7 @@ from __future__ import division
  from __future__ import print_function
  import abc
  +import six
  from typing import Any, Generic, Text, Type, TypeVar
  from google3.third_party.tink.proto import tink_pb2
  @@ -35,6 +36,7 @@ def deserialize_key_data(serialized_proto):
  return key_data
  +@six.add_metaclass(abc.ABCMeta)
  class KeyManager(Generic[P]):
  """Generates keys and provides primitives for the keys.
  @@ -45,8 +47,6 @@ class KeyManager(Generic[P]):
  buffer.
  """
  - __metaclass__ = abc.ABCMeta
  -
  @abc.abstractmethod
  def primitive_class(self) -> Type[P]:
  """The class of the primitive it uses. Used for internal management."""
  