Remove all expired entries from the cache.

def _purge_expired(self):
        """
        Remove all expired entries from the cache.
        """
        time_horizon = time.time() - self._keep_time
        new_cache = {}
        for (k, v) in self._cache.items():
            if v.timestamp > time_horizon:
                new_cache[k] = v
        self._cache = new_cache