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/Objects/weakrefobject.c b/Objects/weakrefobject.c
index 6ceed73..83e63fc 100644
--- a/Objects/weakrefobject.c
+++ b/Objects/weakrefobject.c
@@ -167,7 +167,7 @@
 	if (nameobj == NULL)
 		PyErr_Clear();
 	else if (PyUnicode_Check(nameobj))
-		name = PyUnicode_AsString(nameobj);
+		name = _PyUnicode_AsString(nameobj);
         PyOS_snprintf(buffer, sizeof(buffer),
 		      name ? "<weakref at %p; to '%.50s' at %p (%s)>"
 		           : "<weakref at %p; to '%.50s' at %p>",