On one hand I agree that it would be nice to get rid of these implementation details that prevent some C functions/methods to accept keyword args, but on the other hand I'm not sure that changing them all is the right thing to do.
For some functions/methods being able to pass keyword args make the code more readable/flexible, but for some other there's no real gain.
It also seems to me that since the arguments where only positional, not much thought went into choosing an appropriate name for these arguments.
For example str.join() is documented as str.join(iterable), and the C function calls the argument 'data'.
If we use those names, we won't have a chance to fix them later, so we should be careful before doing a mass-update.