Do not make object inheritance

This patch removes these lintian warnings:

Warning R0205: Class 'X' inherits from object,
can be safely removed from bases in python3 (useless-object-inheritance)

offlineimap/utils/const.py

@@ -6,7 +6,7 @@
 import copy
 
 
-class ConstProxy(object):
+class ConstProxy():
     """Implements read-only access to a given object
     that can be attached to each instance only once."""