Add ruff to repo to lint Python scripts.

What is the problem or limitation you are having?

This repository contains Python code, but we are not enforcing Ruff style configurations on that code.

Describe the solution you'd like

Add a ruff configuration to pyproject.toml, and a pre-commit rule to enforce ruff formatting.

Describe alternatives you've considered

Additional context

The initial ruff linting ruleset can be copied from Toga's configuration.

After adding the configuration, run pre-commit run --all to ensure style is enforced on all existing code. This will almost certainly result in some formatting updates.