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/mbnames.py

@@ -78,7 +78,7 @@ def writeIntermediateFile(accountname):
         _mbnames.write()
 
 
-class _IntermediateMbnames(object):
+class _IntermediateMbnames():
     """mbnames data for one account."""
 
     def __init__(self, accountname, folder_root, mbnamesdir, folderfilter,
@@ -120,7 +120,7 @@ class _IntermediateMbnames(object):
                 json.dump(itemlist, intermediateFD)
 
 
-class _Mbnames(object):
+class _Mbnames():
     def __init__(self, config, ui, dry_run):
 
         self._config = config