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/errors.c b/Python/errors.c
index 5ee6255..9c9eb2f 100644
--- a/Python/errors.c
+++ b/Python/errors.c
@@ -709,7 +709,7 @@
 			if (moduleName == NULL)
 				PyFile_WriteString("<unknown>", f);
 			else {
-				char* modstr = PyUnicode_AsString(moduleName);
+				char* modstr = _PyUnicode_AsString(moduleName);
 				if (modstr &&
 				    strcmp(modstr, "builtins") != 0)
 				{