因为文档说的是：

动态地将抽象方法添加到类中，或尝试
创建方法或类后，立即修改其抽象状态，因此不支持
。 abstractmethod（）仅影响使用常规继承派生的子类
。使用ABC的register（）方法注册
的虚拟子类不受影响。

Dynamically adding abstract methods to a class, or attempting to modify the abstraction status of a method or class once it is created, are not supported. The abstractmethod() only affects subclasses derived using regular inheritance; "virtual subclasses" registered with the ABC’s register() method are not affected.

仅当以下情况调用元类时：类已定义。当抽象方法将类标记为抽象时，状态以后将不会更改。

A metaclass is only called when a class is defined. When abstractmethod has marked a class as abstract that status won't change later.