docs: def sync(self):
          """Sync the timeout index entry with the shelf."""
          if self.writeback and self.cache:
              super(_TimeoutMixin, self).__delitem__(self._INDEX)
              super(_TimeoutMixin, self).sync()
              self.writeback = False
              super(_TimeoutMixin, self).__setitem__(self._INDEX, self._index)
              self.writeback = True
          if hasattr(self.dict, 'sync'):
              self.dict.sync()