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/future.c b/Python/future.c
index a5bee0c..e775384 100644
--- a/Python/future.c
+++ b/Python/future.c
@@ -20,7 +20,7 @@
 	names = s->v.ImportFrom.names;
 	for (i = 0; i < asdl_seq_LEN(names); i++) {
                 alias_ty name = (alias_ty)asdl_seq_GET(names, i);
-		const char *feature = PyUnicode_AsString(name->name);
+		const char *feature = _PyUnicode_AsString(name->name);
 		if (!feature)
 			return 0;
 		if (strcmp(feature, FUTURE_NESTED_SCOPES) == 0) {