clarify Final reassignment semantics

[KotlinIsland]

https://discuss.python.org/t/clarify-final-assignment/106878

[carljm]

I think this is a superior rule, but I would be likely to think so, since it's the rule ty already implements 😆

I think this PR needs some improvements, but I'm in favor of the change.

[carljm]

I'm not sure I agree that it should be required to enforce exhaustive assignment. Why should this be enforced for a Final name, but not for other names?

[KotlinIsland]

i would expect a type checker to enforce this for all names. but this change is only regarding Final, which to me is even more "static" than normal variables, that a Final name without a value should always be impossible