[ty] Use try-call-dunder pattern for `__class_getitem__` (#23066)
## Summary
I think `__class_getitem__` is a bit different than (e.g.)
`__contains__` because it's called on the class itself, not instances...
So given `Identity[0]` where `Identity` is a class, using
`try_call_dunder` would call `member_lookup_with_policy` with
`NO_INSTANCE_FALLBACK`, which would look at the metaclass, but not
`Identity` itself.
I added `try_call_dunder_on_class` for consistency which calls `member`
(without `NO_INSTANCE_FALLBACK`), though as far as this PR is concerned,
I think our current behavior was ok?
Closes https://github.com/astral-sh/ty/issues/190.
---------
Co-authored-by: David Peter <mail@david-peter.de>
