Add dummy class for Constant

Author: dflook

src/python_minifier/ast_compat.py

@@ -53,6 +53,7 @@ def __new__(cls, *args, **kwargs):
     'AsyncFunctionDef',
     'AsyncWith',
     'Bytes',
+    'Constant',
     'DictComp',
     'Exec',
     'ListComp',