--warn-return-any should not complain on NotImplemented when using abc

I'm using `abc.abstractmethod`:
```python
@abc.abstractmethod
def encode(self) -> bytes:
    return NotImplemented
```

And I get a warning: `warning: Returning Any from function declared to return "bytes"`

I don't think something is wrong with the code, it's totally normal way to declare and describe an interface and mypy shouldn't complain about it. 
