[CIVIS-5256] MAINT switch to `pyproject toml` for packaging

[jacksonlee-civis]

It's time to modernize civis-python for packaging, by migrating from the legacy setup.py-based approach to pyproject.toml.

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• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [x] If applicable, unit tests have been added and/or updated
• [x] The CircleCI builds have all passed