every other ast object has a dict, so I think AST should, too

Lib/test/test_ast.py

@@ -195,9 +195,6 @@ class AST_Tests(unittest.TestCase):
         with self.assertRaises(AttributeError):
             x.vararg
 
-        with self.assertRaises(AttributeError):
-            x.foobar = 21
-
         with self.assertRaises(AttributeError):
             ast.AST(lineno=2)