Allow adding a kwonly arg with "expected" in wraps

[aulemahal]

Hi!

I think it could be nice to allow the addition of a keyword-only arg when wrapping a function and using expected.

As wraps and update_wrapper only pass the items of expected directly to FunctionBuilder.add_arg, it could be a simple matter of allowing 3 elements tuples, i.e. : (name, default, kwonly), where kwonly is a boolean.

I might have time to suggest a PR in the next week.