feat: migrate to pyproject.toml

[rinarakaki]

Tracking issue googleapis/google-cloud-python#13171

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Fixes #<issue_number_goes_here> 🦕

[parthea]

Thanks @rinarakaki!

[parthea]

Adding do not merge until #729 is merged to avoid delaying the Python 3.13 release.

[rinarakaki]

@parthea thanks for the review and the merge!