Add a functools.cache variant for methods to avoid keeping instances alive

functools.cache, when applied to a method, caches the self argument. This can also keep the inst 5F0A ance alive long after it is no longer needed. See for example this discussion.

Following the same API as singledispatch and singledispatchmethod, we should consider adding a cachemethod decorator that either avoids caching self, or uses a weakref for it.