message: Fix import depth issue.
diff: @@ -33,7 +33,7 @@ from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  -import six
  +from six import PY3
  from google.protobuf import text_format as _text_format
  from google.protobuf.message import DecodeError
  @@ -192,7 +192,7 @@ class TocoConverter(object):
  print("Ignore 'tcmalloc: large alloc' warnings.")
  if not isinstance(file_content, str):
  - if six.PY3:
  + if PY3:
  file_content = file_content.decode('utf-8')
  else:
  file_content = file_content.encode('utf-8')
  