chore: fix flake8 lint on python >= 3.8

Author: lihsai0

qiniu/services/storage/uploaders/_default_retrier.py

@@ -198,9 +198,11 @@ def get_default_retrier(
             record_exists_handler=progress_record.exists
         ))
 
-        def handle_change_region(_):
+        def _handle_change_region(_):
             progress_record.delete()
 
+        handle_change_region = _handle_change_region
+
     retry_policies += [
         EndpointsRetryPolicy(skip_init_context=True),
         RegionsRetryPolicy(