Build system cleanup

[clbarnes]

There is some configuration which is a bit scattered; IMO it would be useful to centralise.

• Currently, the codespell configuration is split between pyproject.toml and .codespellrc. I recommend putting it in pyproject.toml.
• Currently, codespell checks a number of directories it shouldn't, including .venv, .tox, and _build; we should add these to the ignore section of the config.
• Currently, tox is used to manage an environment in which to run pytest. However, uv can do this for us; tox would be useful if we were running jobs across multiple environments, which we're not. I recommend scrapping tox entirely and just using uv run pytest.
• Currently, commands for common dev tasks (building, linting, testing etc.) are tool-specific and scattered in various config files. I propose putting them all in a justfile. The only downside of just is that it isn't easily available on readthedocs. The downside of make is that using make as a task runner is sort of an abuse of what make is for.