-Fix ABC deprecations, targeted for removal in python 3.8

Author: narfman0

flask_restplus/reqparse.py

@@ -4,7 +4,7 @@
 import decimal
 import six
 
-from collections import Hashable
+from collections.abc import Hashable
 from copy import deepcopy
 from flask import current_app, request
 

flask_restplus/swagger.py

@@ -5,7 +5,8 @@
 import re
 
 from inspect import isclass, getdoc
-from collections import OrderedDict, Hashable
+from collections import OrderedDict
+from collections.abc import Hashable
 from six import string_types, itervalues, iteritems, iterkeys
 
 from flask import current_app