bpo-42374: Allow unparenthesized walrus in genexps (GH-23319) (GH-23329)

This fixes a regression that was introduced by the new parser.

(cherry picked from commit cb3e5ed071)

Lib/test/test_named_expressions.py

@@ -513,6 +513,15 @@ spam()"""
             self.assertEqual(nonlocal_var, None)
         f()
 
+    def test_named_expression_scope_in_genexp(self):
+        a = 1
+        b = [1, 2, 3, 4]
+        genexp = (c := i + a for i in b)
+
+        self.assertNotIn("c", locals())
+        for idx, elem in enumerate(genexp):
+            self.assertEqual(elem, b[idx] + a)
+
 
 if __name__ == "__main__":
     unittest.main()