__wrapped__ is always set on unbound local proxy

[davidism]

Doctest calls inspect.unwrap, which does getattr(obj, "__wrapped__"), which would fail if the proxy was anything but a function. In Flask 2.2, the proxies no longer use functions, so users started reporting that their doctests were failing.

Now the proxy always records what it's wrapping, and returns that when accessing __wrapped__ when unbound. When bound, the attribute access is always passed through to the real object, rather than always returning the proxy's value.

fixes #2485