Improve package setup (dev requirements and project_urls)

[Jacob-Stevens-Haas]

I was hoping to help with onc package development given that I'm working on a 3rd party library, tehom. There's a few things that made it tricky off the bat:

• No requirements-dev.txt or install option for dev dependencies
• PyPI does not link to github (project_urls in setup.py).
• setup.py deprecated.
• Formatter and linter missing.
• Add GitHub Actions for CI #15

I can PR these issues; are you amenable to moving from the deprecated setup.py to pyproject.toml format a la PEP 621, 517, 518?