Python 3.12 support, various developer-side QoL improvements

[leroyvn]

This PR brings several developer quality-of-life improvements, as well as official Python 3.12 support. Changes are as follows:

• Official Python 3.12 support. Python 3.12 is now part of our CI matrix, which makes it officially supported!
• Move from PDM to Rye for project management. Rye, despite its experimental status, is now reliable and has shown its superiority to other project management tools when it comes to managing dependencies. I've been using it for a while now, moving from PDM, and I never regretted it.
• Drop Nox for testing. The point of using Nox was to be able to run locally the full test matrix (with the Python version as a dimension). This turned out to be rather useless, since the CI doesn't even rely on it. Therefore, I'm retiring this: we test locally against a single Python version, the CI does the rest, and when there is a bug, Rye makes it extremely easy to pin a different Python version for debugging.
• Drop Conda development environment support. With Rye as the project management solution, virtual environment management becomes very simple. I no longer feel the need to use Conda for this, so I don't see the point of maintaining additional lock files.
• Use Ruff to format code. The Ruff formatter is basically Black with minor changes. To simplify pre-commit hooks, I switched to the Ruff formatter. This doesn't mean, however, than Black can't be used to format our code.
• Drop Copier template. I decided to stop sourcing project tooling updates from a Coper template (I realized maintaining it required more work than it saved me).

To do:

• Update maintainer documentation
• Update change log
• Pimp this PR description