Use the new TypeIs feature to update the notna/notnull/isna/isnull type guards

[gandhis1]

I originally contributed the annotations for these methods in #339. That PR has a bunch of context about how the fact that TypeGuard only narrows in the positive case, while the methods should narrow in both the positive and negative case.

TypeIs was recently introduced as a PEP and mypy added support for it, I suspect pyright as well: https://mypy-lang.blogspot.com/2024/04/mypy-110-released.html

This would allow narrowing in the negative case and fix the type narrowing semantics of these methods.