Here's an updated patch. I've reverted to the name "simplegeneric" and
documented the limitation around ABCs (I've tried to give an explanation
why it's there, as well as a hint on now to work around the limitation -
let me know if I'm overdoing it, or the text needs rewording).

I've also fixed the wrapper to use update_wrapper to copy the
attributes. That way, there's no duplication.