Generate a 3-tuple `(partname, content_type, blob)` for each of the
        serialized parts in the package.

def iter_sparts(self):
        """
        Generate a 3-tuple `(partname, content_type, blob)` for each of the
        serialized parts in the package.
        """
        for spart in self._sparts:
            yield (spart.partname, spart.content_type, spart.blob)