nb_bound_method: add introspection attributes and fix surprising behavior around wrapper descriptors

[oremanj]

The introspection attributes __self__ and __func__ provide better interface compatibility with regular Python bound methods, and can be cheaply supported by adding new entries in the existing member list.

The previous wholesale forwarding of attribute accesses to the underlying nb_func produced surprising behavior when explicitly invoking dunder methods: obj.method() would work but obj.method.__call__() would not (it would need obj.method.__call__(obj)). Same thing for __repr__, __hash__, etc. Fix this by checking the 'normal' attribute path before delegating to the contained method, except for __doc__ and __module__ where we want nb_func's version even though the attribute does exist in nb_bound_method's class dict as well. This is necessary to support __self__ and __func__ but I think it's a good idea regardless.

[oremanj]

CI failure is the usual windows stderr-capturing flake.

[wjakob]

Nice, I like it -- thanks!