===================
- Reimplement ``LRUCache`` and ``TTLCache`` using
``collections.OrderedDict``. Note that this will break pickle
compatibility with previous versions.
- Fix ``TTLCache`` not calling ``__missing__()`` of derived classes.
- Handle ``ValueError`` in ``Cache.__missing__()`` for consistency
with caching decorators.
- Improve how ``TTLCache`` handles expired items.
- Use ``Counter.most_common()`` for ``LFUCache.popitem()``.