bpo-36052: Raise a SyntaxError when assigning a value to __debug__ with := (GH-11958)

Trying to assign a value to __debug__ using the assignment operator is supposed to fail, but
a missing check for forbidden names when setting the context in the ast was preventing this behaviour.

Python/ast.c

@@ -1084,7 +1084,7 @@ set_context(struct compiling *c, expr_ty e, expr_context_ty ctx, const node *n)
                 return 0;
             break;
         case Name_kind:
-            if (ctx == Store) {
+            if (ctx == Store || ctx == NamedStore) {
                 if (forbidden_name(c, e->v.Name.id, n, 0))
                     return 0; /* forbidden_name() calls ast_error() */
             }