Fix dataclass package issue for python 3.6.

(cherry picked from commit b17ac2317086531ecd3279ff5c47a6944e32838e)

setup.py

@@ -5,7 +5,7 @@ REQUIREMENTS = [
     'absl-py==0.9.0',
     'allennlp==1.2.0',
     'conllu==2.3.2',
-    'dataclasses==0.5',
+    'dataclasses;python_version<"3.7"',
     'dataclasses-json==0.5.2',
     'joblib==0.14.1',
     'jsonnet==0.15.0',