support a `notabstract` decorator

[KotlinIsland]

class A(ABC):
    @abstractmethod
    @classmethod
    def foo(): ...

    @notabstract
    @classmethod
    def bar(cls):
        print("hi")
A.foo()  # should be an error
A.bar()  # should be okay

maybe this could work for instances too, idk

related:

• #752