[functools] Allow method override with @cache decorator

[emmanuel-ferdman]

PR Summary

When you use the @cache decorator on a method and then override that method in a subclass with a more specific return type, mypy incorrectly rejects it. For example, if a parent class has a cached method returning Fruit and a child class overrides it to return Apple (a subclass of Fruit), this should be valid but wasn't. The fix changes _lru_cache_wrapper to use a covariant type parameter, which is safe because the type only appears in return positions. This matches how cached_property already works in the same file.

Fixes #15139.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Thanks!

[ktbarrett]

@emmanuel-ferdman Saw this randomly browsing the repo. Just wanted to make sure you know that using @cache on methods keeps all instances alive forever (or until the cache is cleared).