create NameConstant AST class for None, True, and False literals (closes #16619)
diff --git a/Parser/Python.asdl b/Parser/Python.asdl
index c24d840..0d373b0 100644
--- a/Parser/Python.asdl
+++ b/Parser/Python.asdl
@@ -1,4 +1,4 @@
--- ASDL's five builtin types are identifier, int, string, bytes, object
+-- ASDL's six builtin types are identifier, int, string, bytes, object, singleton
module Python
{
@@ -69,8 +69,8 @@
| Num(object n) -- a number as a PyObject.
| Str(string s) -- need to specify raw, unicode, etc?
| Bytes(bytes s)
+ | NameConstant(singleton value)
| Ellipsis
- -- other literals? bools?
-- the following expression can appear in assignment context
| Attribute(expr value, identifier attr, expr_context ctx)