--- a/web/views/boxes.py	Wed Sep 16 11:28:07 2015 +0200
+++ b/web/views/boxes.py	Wed Sep 16 13:57:21 2015 +0200
@@ -30,6 +30,8 @@
 
 from warnings import warn
 
+from six import add_metaclass
+
 from logilab.mtconverter import xml_escape
 from logilab.common.deprecation import class_deprecated
 
@@ -231,9 +233,9 @@
 
  # helper classes ##############################################################
 
+@add_metaclass(class_deprecated)
 class SideBoxView(EntityView):
     """helper view class to display some entities in a sidebox"""
-    __metaclass__ = class_deprecated
     __deprecation_warning__ = '[3.10] SideBoxView is deprecated, use RsetBox instead (%(cls)s)'
 
     __regid__ = 'sidebox'