A quick note about the extension mechanism.  Currently the only way to extend PyArg_Parse* is via O&.  Therefore, any extended type you add will use O&, and will have a "converter".  So internally all I did was say "if the parameter has a converter, ignore the type and use the O& format unit".

This may not be the perfect extension API, if we change Clinic to using some new yet-to-be-defined API for argument parsing.  But I suspect it's pretty close.  Extension types will need a conversion function from PyObject to their type, they'll need a way of defining the C default value, and they'll need a way of cleaning up afterwards, all of which I already do.  So I guess I'm interested in feedback on the extension API too.