Add static analysis support to _util.Final

[jakkdl]

See discussion in #2668 (comment)

@final was added in 3.8, so it would be good to make classes decorated with _util.Final also be treated as @final by type checkers. I didn't find a good way of applying both with a single decorator or metaclass, so unless I missed a way we might have to have both on all classes. Or possibly skip one of them.

[A5rocks]

Might be a nice case for just a lint to ensure we have both? I can't think of a solution here unfortunately.

[TeamSpen210]

There is actually a better way, but it'd require some churn. By using __init_subclass__, we can implement _util.Final as a decorator. Then we can just make it an alias to the typing version if TYPE_CHECKING. A decorator's better anyway since we don't have to worry about metaclass conflicts.

The biggest downside is that we have to remember to use @final alongside NoPublicConstructor, since I can't think of a way to make the latter a decorator. We could fairly easily add a check to the test_exports logic to look for these cases though.

I don't think we need to care about backwards compatibility, since these are both private.

[jakkdl]

Sounds like a decent enough solution to me 👍