Modernize pkg

[deppen8]

This PR is designed to update the tooling around the package to better fit modern practices.

A brief summary of some of the major changes:

• Support only Python 3.8 and above
• Switch from setup.py to pyproject.toml
• Use src/package_name layout
• Switch to a calendar versioning scheme
• Use hatch for environment management, running commands, and building the package
• Change git repo default branch to main

I plan to create another PR to add more docs before creating a new release to PyPI + conda-forge.