Migrate to `pyproject.toml`, update min Python to 3.9, consolidate dev workflows

[VeckoTheGecko]

Changes:

• Migrate to pyproject.toml
• Update min Python version to 3.9
• Add pypi metadata classifiers
• Add contributing guide
• Consolidate installation (for users, testing and docs)
• Dependabot config
• Fix conf.py
• Clean up tests.yml
• Remove circleci
• Add tests.utils to consolidate test path parameters

Can we add pre-commit CI* to this project, as well as set up docs building for PRs via RTD**?

* In preparation for when we add pre-commit config in next PR
** I think this is a better way to check that the docs build. It allows previewing the docs as well (which I don't think the circleci build did).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 54.65%. Comparing base (ac3f2c5) to head (f0eed7a). Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #209      +/-   ##
==========================================
- Coverage   54.90%   54.65%   -0.25%     
==========================================
  Files           9       10       +1     
  Lines        1692     1685       -7     
==========================================
- Hits          929      921       -8     
- Misses        763      764       +1     

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[jklymak]

So what is the plan here? Should we just merge this and see if it works, and then fix any issues?

[jklymak]

Only question I had was the empty __init__.py file in tests/?

[jklymak]

... also - what is the plan for changing the files to ruff? Is that a second PR? I guess I thought that would come before this PR, not after.

[VeckoTheGecko]

So what is the plan here? Should we just merge this and see if it works, and then fix any issues?

Sounds good. Any particular issues that you're concerned about?

Only question I had was the empty __init__.py file in tests/?

This makes tests a package, allowing us to do from tests.utils import ... (consolidating LIBRARY_DIR to be stored in one place).

... also - what is the plan for changing the files to ruff? Is that a second PR? I guess I thought that would come before this PR, not after.

I think this PR first and then #210 would be good. Once this is merged, #210 should only be 2 commits so shouldn't be a need to rebase