Fix unicode attribute error if typing installed on py2.7 (#363)

src/drf_yasg/inspectors/field.py

@@ -2,6 +2,7 @@ import datetime
 import inspect
 import logging
 import operator
+import sys
 import uuid
 from collections import OrderedDict
 from decimal import Decimal
@@ -489,6 +490,9 @@ hinting_type_info = [
     (datetime.date, (openapi.TYPE_STRING, openapi.FORMAT_DATE)),
 ]
 
+if sys.version_info < (3, 0):
+    hinting_type_info.append((unicode, (openapi.TYPE_STRING, None)))
+
 if typing:
     def inspect_collection_hint_class(hint_class):
         args = hint_class.__args__