docs: def allowed_values(self):
          """Return a list of allowed values."""
          if self._allowed_values is None:
              self._allowed_values = ValueList()
              for val in self.scraper._fetch_allowed_values(self):
                  if isinstance(val, DimensionValue):
                      self._allowed_values.append(val)
                  else:
                      self._allowed_values.append(DimensionValue(val,
                                                                 Dimension()))
          return self._allowed_values