Modernize packaging and change license

[felddy]

🗣 Description

• Modernize packaging
• Change license to MIT
• Unpin testing dependencies

💭 Motivation and context

• Standardization with my other projects.
• Modernization so the wheels don't fall off.
• Making less work for future me.

🧪 Testing

CI

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a few goals - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• All new and existing tests pass.