Improvements and housekeeping for 2024-W15

[khaeru]

• Use Pint's documented approach to cache registry instantiation.
• Actually test against the earliest supported Pint version shown in pyproject.toml.
• Add Python 3.12 to classifiers.
• Replace black with ruff for code formatting.
• Use latest ruff, mypy.
• Bump GitHub Actions used in workflows, and add dependabot config to update these automatically in the future.