Sent a PR with the patch.

Nick, I tried your `__set_name__` proposal to get an earlier error in case of an object with slots, but it has the downside that Python seems to always raise a new chained exception if `__set_name__` raises any exception. So instead of getting a clear error, you get an opaque one about "error raised when calling __set_name__ on...", and you have to scroll up to see the real error message. I felt that this was too much usability regression and not worth the benefit of raising the error sooner. Let me know if you feel otherwise.