create NameConstant AST class for None, True, and False literals (closes #16619)
diff --git a/Include/asdl.h b/Include/asdl.h
index 6bf618f..0789ad8 100644
--- a/Include/asdl.h
+++ b/Include/asdl.h
@@ -5,6 +5,7 @@
 typedef PyObject * string;
 typedef PyObject * bytes;
 typedef PyObject * object;
+typedef PyObject * singleton;
 
 /* It would be nice if the code generated by asdl_c.py was completely
    independent of Python, but it is a goal the requires too much work