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/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 45494dd..3ee3bf4 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -788,7 +788,7 @@
 	result = PyUnicode_FromFormat(FORMAT, (TYPE)1);	\
 	if (result == NULL)				\
 		return NULL;				\
-	if (strcmp(PyUnicode_AsString(result), "1")) {	\
+	if (strcmp(_PyUnicode_AsString(result), "1")) {	\
 		msg = FORMAT " failed at 1";		\
 		goto Fail;				\
 	}						\