`stubtest` produce no error when `@abstractmethod` is omitted from stubs

[sobolevn]

If I have this runtime implementation:

from abc import abstractmethod

class A:
    @abstractmethod
    def some(self) -> None:
        raise NotImplementedError

And this stub:

class A:
    def some(self) -> None: ...

There's no error.

Why is that important?
Because with the stub like above, mypy will allow to create an instance of A.
But, mypy should not be allow to do that.

I think it is a bug that should be fixed. And it is a pretty easy one.

[AlexWaygood]

I used a stubtest patch along these lines to work on this typeshed PR in March:

• stdlib: Add several missing @abstractmethod decorators typeshed#7443

But I never got round to writing tests and submitting a PR to mypy. I'm glad you're picking it up :D