remove (already commented out) pip installs in py scripts; move to pyproject.toml/requirements.txt

[chunhochow]

Lines like https://github.com/sfcta/validation_workbook/blob/main/transit/script/transit.py#L6-L19 should be removed. Add all python requirements to pyproject.toml under dependencies (https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#dependencies-and-requirements) or equivalent like a requirements.txt file (https://pip.pypa.io/en/latest/user_guide/#requirements-files). (though the latter seems to be deprecated in favour of pyproject.toml when it's not pinned dependencies)