Fix narrowing with final type objects (#20743)
Follow up feature request from here:
https://github.com/python/mypy/pull/20675#discussion_r2747054587
Preserves correct behaviour on this test case:
https://github.com/python/mypy/pull/20675/files#diff-e3de7a75a8a107b4f462b164cdf4945d50505c5e9f7092b753c4add0c01530bbR3021-R3037
