Return |_SerializedRelationships| instance populated with
        relationships for source identified by *source_uri*.

def _srels_for(phys_reader, source_uri):
        """
        Return |_SerializedRelationships| instance populated with
        relationships for source identified by *source_uri*.
        """
        rels_xml = phys_reader.rels_xml_for(source_uri)
        return _SerializedRelationships.load_from_xml(
            source_uri.baseURI, rels_xml)