Ref #19274

This is a bit ugly. But I propose to have this "hot-fix" until we have a
proper overhaul of instance vs class variables. To be clear: attribute
access already works correctly (on both `P` and `Type[P]`), but
subtyping returns false because of
```python
                elif (IS_CLASSVAR in subflags) != (IS_CLASSVAR in superflags):
                    return False
```