Could you add a comment on why you're calling PyUnicode_FromString and then throwing away the result? I believe it's so you'll get the same error checking as PyUnicode_FromString, but it's sufficiently tricky that I think it deserves a comment.

Now that I think about it, having done the conversion in PyUnicode_FromString, why not use:
buf[0] = PyUnicode_AS_UNICODE(s)[0];
? This way you skip the cast and you know for a fact you're using the same interpretation as PyUnicode_FromString, having used its output.

Also, since you know the size you may as well call PyUnicode_FromStringAndSize. It's trivially faster and makes the intent clearer, I think.