Add Python 3.13

[bgilbert]

Adding a new Python release

• Update Git main
  • [x] git checkout main
  • [x] In pyproject.toml, add classifier for new Python version and update tool.black.target-version
  • [x] Add new Python version to lists in .github/workflows/python.yml
  • [x] Commit and open a PR
  • [x] Merge the PR when CI passes
  • [x] Add new Python jobs to branch protection required checks
• [ ] Update MacPorts package
• [ ] Update website: Python 3 versions in download/index.md