Rename PyUnicode_AsString -> _PyUnicode_AsString and
PyUnicode_AsStringAndSize -> _PyUnicode_AsStringAndSize to mark
them for interpreter internal use only.

We'll have to rework these APIs or create new ones for the
purpose of accessing the UTF-8 representation of Unicode objects
for 3.1.
diff --git a/Parser/tokenizer.c b/Parser/tokenizer.c
index 487405f..e2da3e5 100644
--- a/Parser/tokenizer.c
+++ b/Parser/tokenizer.c
@@ -391,7 +391,7 @@
 	}
 	if (PyUnicode_CheckExact(bufobj))
 	{
-		buf = PyUnicode_AsStringAndSize(bufobj, &buflen);
+		buf = _PyUnicode_AsStringAndSize(bufobj, &buflen);
 		if (buf == NULL) {
 			goto error;
 		}