`cls` in classmethods of derived classes always refers to the parent class (rather than the derived one)

Easier to explain with an example:

```python
class Foo:
    @classmethod
    def get_an_instance(cls):
        return cls()

class Bar(Foo):
    pass

my_bar = Bar.get_an_instance()
```

In VSCode w/ Python plugin 2018.8.0, `my_bar` is typed as a `Foo`, while I'd expect it to be typed as a `Bar`.