Return a new ``<Override>`` element with attributes set to parameter
        values.

def new(partname, content_type):
        """
        Return a new ``<Override>`` element with attributes set to parameter
        values.
        """
        xml = '<Override xmlns="%s"/>' % nsmap['ct']
        override = parse_xml(xml)
        override.set('PartName', partname)
        override.set('ContentType', content_type)
        return override