patches/DCWorkflow.py: add the missing library.

product/ERP5Type/patches/DCWorkflow.py

@@ -40,6 +40,7 @@ from Products.DCWorkflow.Guard import Guard, _checkPermission
 from Products.DCWorkflow.States import StateDefinition
 from Products.DCWorkflow.Variables import VariableDefinition
 from Products.DCWorkflow.Worklists import WorklistDefinition
+from types import StringTypes
 from zLOG import LOG, INFO, WARNING
 # Libraries related to showAsXML
 from lxml import etree