modify mock task

Signed-off-by: Jose Borreguero <borreguero@gmail.com>

django_remote_submission/tasks.py

@@ -450,7 +450,7 @@ def delete_key_from_server(
 
 # DEBUG: remove this mock task
 @shared_task
-def clean_expired_sessions():
+def clean_expired_sessions() -> None:
     r"""Print a silly message"""
     current_time = datetime.now().strftime("%H:%M:%S")
     print(