Is it possible to use Argument Clinic for third party extensions? If yes, I
suggest to use it.

It would also be nice if Cython could use it automatically. So we can
suggest to use Cython. I don't know well Cython, maybe your idea is very
dumb :-)

Serhiy Storchaka added the comment:
> I suggest to keep it internal. It's meaning may be changed in future. I
have some ideas about making it more convenient and faster (but I'm not
sure that they would work).

Ah? Can you please elaborate your secret plan? :-)