def _cleanup(self):
        """Remove any stale files from the session storage directory"""
        for filename in os.listdir(self._storage_dir):
            file_path = path.join(self._storage_dir, filename)
            file_stat = os.stat(file_path)
            evaluate = max(file_stat.st_ctime, file_stat.st_mtime)
            if evaluate + self._duration < time.time():
                LOGGER.debug('Removing stale file: %s', file_path)
                os.unlink(file_path)
Remove any stale files from the session storage directory