Infrastructure update

[mkelley]

Remove sbpy.cfg, moving material to pyproject.toml

Remove astropy testrunner, addresses #423

[github-actions]

Thank you for your contribution to sbpy, an Astropy affiliated package! 🌌 This checklist is meant to remind the package maintainers who will review this pull request of some common things to look for.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the sbpy coding guidelines?
• Are tests added/updated as required? If so, do they follow the Astropy testing guidelines?
• Are docs added/updated as required? If so, do they follow the Astropy documentation guidelines?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions. Also see instructions for rebase and squash.
• Did the CI pass? If no, are the failures related?
• Is a change log needed? If yes, did the change log check pass? If no, add the "no-changelog-entry-needed" label.
• Is a milestone added?