Add dummy class for AsyncFor

Author: dflook

src/python_minifier/ast_compat.py

@@ -49,6 +49,7 @@ def __new__(cls, *args, **kwargs):
 # Create a dummy class for missing AST nodes
 for _node_type in [
     'AnnAssign',
+    'AsyncFor',
     'AsyncFunctionDef',
     'AsyncFunctionDef',
     'AsyncWith',