Add missing import of PropertySheet within Constraint mixin

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@41008 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/mixin/constraint.py

@@ -34,7 +34,7 @@ from Products.ERP5Type.ConsistencyMessage import ConsistencyMessage
 from zope.interface import implements
 from Products.ERP5Type.Core.Predicate import Predicate
 from AccessControl import ClassSecurityInfo
-from Products.ERP5Type import Permissions
+from Products.ERP5Type import Permissions, PropertySheet
 from Products.ERP5Type.Utils import UpperCase, createExpressionContext
 from Products.CMFCore.Expression import Expression