[Python-3000] C API cleanup str

["Martin v. Löwis"]

> One issue with just putting this in the C API docs is that I believe
> (tell me if I'm wrong) that these haven't been kept up to date in the
> struni branch so we'll need to make a lot more changes than just this
> one...

That's certainly the case. However, if we end up deleting the str8
type entirely, I'd be in favor of recycling the PyString_* names
for Unicode, in which case everything needs to be edited, anyway.

Regards,
Martin