Simplify, cleanup package 'meta' so easier to maintain

[brews]

This is about cleanup related to what we found closing #107. We can be a bit more confident in changing things now that tests are running.

The package itself can be a bit hard to maintain in the long run because there are many moving parts. Many of the parts are outdated and should be removed or replaced.

Let's see how much we can simplify the package so it's easier to maintain and we'll also update some things!

• setup.py -> pyproject.toml, build with hatch instead of setuptools. Consolidate as much of the config stuff as we can. But try to keep the original spirit of the package. (PR Meta cleanup #128)
• Consolidate requirements to one requirements.txt for CI, but note dependencies in pyproject.toml. Don't think we require conda for testing...? (PR Meta cleanup #128)
• Swap linting and flake8 stuff for ruff. We can ditch the .cfg file in the repo root. (PR Meta cleanup #128)
• Ditch tox. Don't need it. Likely outdated. (PR Meta cleanup #128)
• Ditch Makefile. Don't need it. Likely outdated. (PR Meta cleanup #128)
• Ditch pytest-runner as this is frowned on these days. Outdated. (PR Meta cleanup #128)
• Update/simplify docs, readthedocs integration? Not as sure on this one. At least it needs to be updated. Test docs in CI? Mike says he's fine ditching it.
• Update release process? Is the previous method better than 'Update CHANGELOG.md and do a github release'?
• Drop Python 2 support officially. (PR Meta cleanup #128)