code: def from_xml(cls, xml_bytes):
          
          root = XML(xml_bytes)
          return cls(root.findtext('Bucket'),
                     root.findtext('Key'),
                     root.findtext('UploadId'))
docs: Create an instance of this from XML bytes.
  
          @param xml_bytes: C{str} bytes of XML to parse
          @return: an instance of L{MultipartInitiationResponse}