better not to compare string with 'is'

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@16928 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Base.py

@@ -1209,7 +1209,7 @@ class Base( CopyContainer,
 
       **kw allows to call setProperty as a generic setter (ex. setProperty(value_uid, portal_type=))
     """
-    if type is not 'string': # Speed
+    if type != 'string': # Speed
       if type in list_types: # Patch for OFS PropertyManager
         key += '_list'
     accessor_name = '_set' + UpperCase(key)