Remove every old-style class.

src/memory/persistent.py

@@ -24,7 +24,7 @@ from logging import debug, info, warning, error
 class MemoryNotInitializedError(Exception):
     pass
 
-class PersistentDict:
+class PersistentDict(object):
     """A dictionary which save persistently each value assigned
     /!\ be careful, each assignment means a database write
     /!\ Memory must be initialised before loading/setting value with instances of this class"""