add default build and test workflows and expand toxenvs

[zacharyburnett]

• use OpenAstronomy reusable workflows for building and testing
• use tox-uv to resolve lowest-direct, instead of minimum_dependencies, for oldestdeps
• add devdeps and xdist tox factors

[zacharyburnett]

template tests seem to be failing at checking that license files are correctly templated; I'll take a look more closely at this