(🎁) Intersections with `@final` types should trigger unreachable errors

[KotlinIsland]

from typing import final

@final
class A: ...
class B: ...

a: A
assert isinstance(a, B)
reveal_type(a)  # note: Revealed type is "__main__.<subclass of "A" and "B">"
print("hi") # no unreachable error

• Relates to TypeVar not narrowed when using isinstance or issubclass on final classes #12163

[NeilGirdhar]

"Intersections with @final types should trigger unreachable errors"… (unless A already inherits from B of course 😄)

[tyralla]

@KotlinIsland: #16330 should fix this issue. Would you like to review it?