docs: 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)