Added required get_user function to UsernameCardnumberPinBackend

cashonly/core/auth.py

@@ -25,3 +25,9 @@ class UsernameCardnumberPinBackend(object):
         if accmgr.check_pin(pin):
             return account.user
         return None
+
+    def get_user(self, user_id):
+        try:
+            return User.objects.get(pk=user_id)
+        except User.DoesNotExist:
+            return None