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

@staticmethod
def _srels_for(phys_reader, source_uri):

    rels_xml = phys_reader.rels_xml_for(source_uri)
    return _SerializedRelationships.load_from_xml(source_uri.baseURI, rels_xml)
