[ty] Infer variance through type[T] (#27534)
## Summary
Previously, variance inference treated `type[T]` as independent of `T`.
As a result, generic classes with writable class-object attributes could
incorrectly be treated as covariant:
```python
class Mutable[T]:
cls: type[T]
def overwrite(value: Mutable[object]) -> None:
value.cls = str
def unsound(value: Mutable[int]) -> None:
overwrite(value) # error: [invalid-argument-type]
```
We now retain the wrapped type variable when inferring variance through
`type[T]`. Return positions remain covariant, parameter positions become
contravariant, and writable public attributes correctly make their
containing class invariant.
