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/traceback.c b/Python/traceback.c
index d569a18..dffce35 100644
--- a/Python/traceback.c
+++ b/Python/traceback.c
@@ -257,10 +257,10 @@
 	while (tb != NULL && err == 0) {
 		if (depth <= limit) {
 			err = tb_displayline(f,
-			    PyUnicode_AsString(
+			    _PyUnicode_AsString(
 				    tb->tb_frame->f_code->co_filename),
 			    tb->tb_lineno,
-			    PyUnicode_AsString(tb->tb_frame->f_code->co_name));
+			    _PyUnicode_AsString(tb->tb_frame->f_code->co_name));
 		}
 		depth--;
 		tb = tb->tb_next;