Turns out there's a bug in the implementation of functools.update_wrapper :P

Added that as a dependency, since this API doesn't make sense until update_wrapper is behaving itself.

The new tests didn't pick it up because they don't use wraps or update_wrapper, they set __wrapped__ directly.

Also, the replacement of the recursion in inspect.signature is incorrect - we want to interleave checks for __signature__ as we recurse through the stack of wrapper functions.