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/_elementtree.c b/Modules/_elementtree.c
index da223c4..767c6ee 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -1303,7 +1303,7 @@
     char *name = "";
 
     if (PyUnicode_Check(nameobj))
-	name = PyUnicode_AsString(nameobj);
+	name = _PyUnicode_AsString(nameobj);
 
     if (strcmp(name, "tag") == 0)
 	res = self->tag;
@@ -2529,7 +2529,7 @@
     char *name = "";
 
     if (PyUnicode_Check(nameobj))
-	name = PyUnicode_AsString(nameobj);
+	name = _PyUnicode_AsString(nameobj);
 
     PyErr_Clear();