duration marshalling hack, thanks Dirk Jonker:
The duration function will return a datetime.timedelta for any ISO8601-complation duration that only contains time. If the duration string contains days, months and/or years, or if it is not compliant, it will simply catch an AttributeError and return None, which it would do without the function too.
Fixed in TYPE_MAP and (UN)MARSHAL_FN for xsd:date, xsd:datetime and xsd:duration datetypes