Get tests running in GitHub Actions

[alexwlchan]

This is the first half of #154

• Replace pip-tools, flake8 and black with uv and ruff as my Python tools of choice (Switch to using uv and ruff as my Python tools of choice alexwlchan#9)
• Add Dependabot config (Add Dependabot config to all my Python repositories alexwlchan#1)
• Add my standard Python GitHub Actions setup, so tests are running in CI again

The one thing that's missing is type checking with Mypy – that's a non-trivial codebase change, so I'll do that in a separate PR.