message: User BinaryIO and TextIO as input type for BinaryKeysetReader and JsonKeysetReader.
diff: @@ -20,12 +20,11 @@ from __future__ import division
  from __future__ import print_function
  import abc
  -import io
  # Special imports
  import six
  -from typing import Union
  +from typing import BinaryIO, TextIO
  from google.protobuf import json_format
  from tink.proto import tink_pb2
  from tink import core
  @@ -52,7 +51,7 @@ class JsonKeysetWriter(KeysetWriter):
  cf. https://developers.google.com/protocol-buffers/docs/encoding
  """
  - def __init__(self, text_io_stream: io.TextIOBase):
  + def __init__(self, text_io_stream: TextIO):
  self._io_stream = text_io_stream
  def write(self, keyset: tink_pb2.Keyset) -> None:
  @@ -84,7 +83,7 @@ class BinaryKeysetWriter(KeysetWriter):
  cf. https://developers.google.com/protocol-buffers/docs/encoding
  """
  - def __init__(self, binary_io_stream: Union[io.BufferedIOBase, io.BytesIO]):
  + def __init__(self, binary_io_stream: BinaryIO):
  self._io_stream = binary_io_stream
  def write(self, keyset: tink_pb2.Keyset) -> None:
  