switch to dependency groups for new package flow

[eleanorjboyd]

For "new package" flow, switch to dependency groups instead of pyproject.toml, dev-requirements.txt going forward. see link for more info (https://ichard26.github.io/blog/2025/04/whats-new-in-pip-25.1/#dependency-groups-pep-735)

[eleanorjboyd]

will await community adoption per @cwebster-99 understanding