Allow accessing `__dict__` on `cls`

[grievejia]

Minimal repro:

class Foo:
    @classmethod
    def foo(cls) -> None:
        cls.__dict__

Expected: No error
Actual: Instance-only attribute __dict__ of class Foo is not visible on the class [missing-attribute]

[grievejia]

Looks like this is the same underlying issue as #123