如何使其他类派生自单例类？

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]

class MyBaseClass(metaclass=Singleton):
    pass

class MyDerivedClass(MyBaseClass):
    pass

base1 = MyBaseClass()
base2 = MyBaseClass()
derived1 = MyDerivedClass()
derived2 = MyDerivedClass()

assert base1 is base2
assert derived1 is derived2
assert base1 is not derived1