use django core implementation of PermissionDenied for tests
import unittest
from django.contrib.auth.models import Group, AnonymousUser
+from django.core.exceptions import PermissionDenied
from django.test import TestCase, RequestFactory
-from rest_framework.exceptions import PermissionDenied, NotFound
+from rest_framework.exceptions import NotFound
from hs_core.hydroshare import resource
from hs_core.hydroshare import users
