Make compatible with Python 2.X

Author: Tim Csitkovics

rest_framework_json_api/serializers.py

@@ -192,8 +192,8 @@ def to_representation(self, instance):
     def _get_field_representation(self, field, instance):
         request = self.context.get('request', None)
         is_included = field.source in get_included_resources(request)
-        if not is_included and isinstance(field, ModelSerializer) and hasattr(instance, f'{field.source}_id'):
-            attribute = getattr(instance, f'{field.source}_id')
+        if not is_included and isinstance(field, ModelSerializer) and hasattr(instance, field.source + '_id'):
+            attribute = getattr(instance, field.source + '_id')
 
             if attribute is None:
                 return None