[CIVIS-5100] SEC add `pip-audit` check

[jacksonlee-civis]

This pull request adds the pip-audit check to CI for potential security vulnerabilities of the Python dependencies that this codebase uses.

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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