#62 Use uv to create venv for specific Python version

[reece]

This PR starts the migration to uv for python dev tooling by creating the venv with uv and using it to specify a specific Python version.

To eval this PR, please follow the steps in the README for developers.

[jsstevenson]

👍 Two general ideas for your consideration:

1. use uv sync instead of uv pip install. The main difference I'm aware of is that uv sync will remove extraneously-installed packages from the environment. This honestly kinda bugs me sometimes but I can see an argument for employing it.

2. use uv run to perform the other entrypoint-based actions (pytest ruff etc). This actually removes the need to have the virtualenv activated (although I think it's generally a good idea to stay within one anyway). This might be out of scope of this ticket though.

oh, also, just from googling... https://github.com/tox-dev/tox-uv ? maybe?