Add a cache interface which supports expirable entries with a changeable
backend for the cache into which they are entered.
The default cache is a `dict` but could swapped for
`cachetools.LRUCache` or any other cache which supports `__get__`,
`__set__`, and `__del__`.
So that consumers can change the use of `CachedObjects` stored in a
cache from:
```
self._cache = {}
...
try:
obj = self._cache[key]
try:
return obj.unwrap(dt)
except Expired:
pass
except KeyError:
pass
...
self._cache[key] = CachedObject(value, new_expiration)
```
to:
```
self._cache = ExpiringCache(LRUCache(maxsize=6))
...
try:
return self._cache.get(key, dt)
except KeyError:
# Get fresh value
...
self._cache.set(key, value, new_expiration)
```
