refactor: break down _cell_magic() into several helper functions to i… (#54)

* refactor: break down _cell_magic() into several helper functions to improve readability

* Fix import

* fix test failures

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* fix type hint for pre 3.9 compatibility

* remove unused variable in test

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: sycai