Build for Python 3.10

[bgilbert]

Adding wheels for a new Python release

• Update Git main
  • [x] git checkout main
  • [x] Add classifier for new Python version to setup.py
  • [x] Add new Python version to lists in .github/workflows/python.yml
  • [x] Commit and push
• Build new wheels
  • [x] Check out a new branch from the most recent release tag
  • [x] Add new Python version to lists in .github/workflows/python.yml, commit, and open a DNM PR
  • [x] Find the workflow run for the PR; download its wheels artifact
  • [x] Close the PR
• [x] In OpenSlide Python checkout, git checkout v<version> && git clean -dxf && mkdir dist
• [x] Copy downloaded wheels from new Python release only into dist directory
• [x] twine upload dist/*
• [x] Upload new wheels to GitHub release
• [x] Update MacPorts package
• [x] Update website: Python 3 versions in download/index.md

[bgilbert]

Build PR in https://github.com/openslide/openslide-python/pull/143; workflow run in https://github.com/openslide/openslide-python/actions/runs/1326796050.

[bgilbert]

MacPorts package bump blocked on 3.10 support in dependencies.