Generate a 4-tuple `(partname, content_type, reltype, blob)` for each
        of the serialized parts in the package.

def iter_sparts(self):
        """
        Generate a 4-tuple `(partname, content_type, reltype, blob)` for each
        of the serialized parts in the package.
        """
        for s in self._sparts:
            yield (s.partname, s.content_type, s.reltype, s.blob)