fix(serializer): restore get_unique_together_constraints method signature

Extracted error message logic to a separate method.

Author: s-aleshin

rest_framework/serializers.py

@@ -1638,7 +1638,7 @@ def get_unique_together_validators(self):
 
             field_names = tuple(source_map[f][0] for f in unique_together)
 
-            constraint = unique_constraint_by_fields.get(tuple(unique_together), None)
+            constraint = unique_constraint_by_fields.get(tuple(unique_together))
             violation_error_message = self._get_constraint_violation_error_message(constraint) if constraint else None
 
             validator = UniqueTogetherValidator(