Add cython language_level hints

Summary: Pull Request resolved: https://github.com/pytorch/fairseq/pull/1147

Differential Revision: D17468447

Pulled By: myleott

fbshipit-source-id: 0dbac04b92c8df74ad991d5e92cd02036d662369

fairseq/data/data_utils_fast.pyx

+# cython: language_level=3
 # Copyright (c) Facebook, Inc. and its affiliates.
 #
 # This source code is licensed under the MIT license found in the

fairseq/data/token_block_utils_fast.pyx

+# cython: language_level=3
 # Copyright (c) Facebook, Inc. and its affiliates.
 #
 # This source code is licensed under the MIT license found in the