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/Python/getargs.c b/Python/getargs.c
index ccd1817..36a6261 100644
--- a/Python/getargs.c
+++ b/Python/getargs.c
@@ -1537,7 +1537,7 @@
 					        "keywords must be strings");
 				return cleanreturn(0, freelist);
 			}
-			ks = PyUnicode_AsString(key);
+			ks = _PyUnicode_AsString(key);
 			for (i = 0; i < len; i++) {
 				if (!strcmp(ks, kwlist[i])) {
 					match = 1;