[3.9] bpo-40939: Fix test_keyword for the old parser (GH-20814)

diff --git a/Grammar/python.gram b/Grammar/python.gram
index 2c350ef..1510683 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -477,7 +477,7 @@
     | 'True' { _Py_Constant(Py_True, NULL, EXTRA) }
     | 'False' { _Py_Constant(Py_False, NULL, EXTRA) }
     | 'None' { _Py_Constant(Py_None, NULL, EXTRA) }
-    | '__new_parser__' { RAISE_SYNTAX_ERROR("You found it!") }
+    | '__peg_parser__' { RAISE_SYNTAX_ERROR("You found it!") }
     | &STRING strings
     | NUMBER
     | &'(' (tuple | group | genexp)