diff -r d394bfcd8c25 -r fa29f3628a1b web/uihelper.py
--- a/web/uihelper.py	Wed Sep 16 11:28:07 2015 +0200
+++ b/web/uihelper.py	Wed Sep 16 13:57:21 2015 +0200
@@ -45,6 +45,7 @@
 """
 __docformat__ = "restructuredtext en"
 
+from six import add_metaclass
 
 from logilab.common.deprecation import deprecated
 from cubicweb.web.views import uicfg
@@ -93,6 +94,7 @@
         super(meta_formconfig, cls).__init__(name, bases, classdict)
 
 
+@add_metaclass(meta_formconfig)
 class FormConfig:
     """helper base class to define uicfg rules on a given entity type.
 
@@ -162,7 +164,6 @@
       inlined = ('use_email',)
 
     """
-    __metaclass__ = meta_formconfig
     formtype = 'main'
     etype = None # must be defined in concrete subclasses
     hidden = ()