Memoize decorate will remove the type hint of the original function

If you put Cache.memoize on a custom function, you will lose the type hint when using that function

```
from diskcache import Cache

cache = Cache(directory=...)

@cache.memorize
def hello(name: str):
    return f"hello {name}"

hello(...) # when you type this, you won't see name as argument in VSCode PyCharm etc ...
```