[stubtest] Better checking of `Final` variables with literal values

[brianschubert]

Given the runtime definition:

FOO = 1

stubtest currently generates an error for this stub:

FOO: Literal[2]  # E: FOO variable differs from runtime type Literal[1]

but not for this:

FOO: Final = 2

It would be helpful if stubtest emitted an error for the latter case as well. See e.g. python/typeshed#15446, python/typeshed#15447 for cases where discrepancies like this have snuck into typeshed.