[Python-3000] C API cleanup str

["Martin v. Löwis"]

>> I agree. We should specify that somewhere, so we have a recorded
>> guideline to use in case of doubt.
> 
> But where? Time to start a PEP for the C API perhaps?

I would put it into the API documentation. We can put a daily-generated
version of the documentation online, just as the trunk documentation is
updated daily.

IMO, a PEP is necessary only for disputed cases. As the C API seems to
get few if any disputes, we just need to record the decisions made.

>> As a consequence, this now can fail because of encoding errors
>> (which it previously couldn't).
> 
> You mean if it were fixed it could fail, right?

Right.

> Have you tried making this particular change and seeing what fails?

No. I suspect most callers pass ASCII, so they should be fine. In
the cases where it really fails, the caller likely meant to create
bytes.

Regards,
Martin