Add old-style class compatible superclass call

* DRF validators are old-style classes on python2.

user_management/api/serializers.py

@@ -11,7 +11,8 @@
 class UniqueEmailValidator(validators.UniqueValidator):
     def filter_queryset(self, value, queryset):
         """Check lower-cased email is unique."""
-        return super(UniqueEmailValidator, self).filter_queryset(
+        return validators.UniqueValidator.filter_queryset(
+            self,
             value.lower(),
             queryset,
         )