docs: def rels_uri(self):
          """
          The pack URI of the .rels part corresponding to the current pack URI.
          Only produces sensible output if the pack URI is a partname or the
          package pseudo-partname '/'.
          """
          rels_filename = '%s.rels' % self.filename
          rels_uri_str = posixpath.join(self.baseURI, '_rels', rels_filename)
          return PackURI(rels_uri_str)