Shrink the cache if necessary, evicting the oldest items.

def _enforce_size_limit(self, capacity):
        """Shrink the cache if necessary, evicting the oldest items."""
        while len(self._cache) > capacity:
            key, value = self._cache.popitem(last=False)
            if self._on_evict is not None:
                self._on_evict(key, value)