Generate a 2-tuple `(source_uri, srel)` for each of the relationships
        in the package.

def iter_srels(self):
        """
        Generate a 2-tuple `(source_uri, srel)` for each of the relationships
        in the package.
        """
        for srel in self._pkg_srels:
            yield (PACKAGE_URI, srel)
        for spart in self._sparts:
            for srel in spart.srels:
                yield (spart.partname, srel)