Things to do

- diagnose python-lint problems with examples/inspect_checkpoint.py!
- * straighten out python requirement (e.g., 3.8 vs. 3.9)
- *** document dependencies among the various configuration files:
  -- .github/workflows/tests.yml
  -- setup.py & mezcla/__init__.py
  -- pyproject.toml [for flit] & tox.ini [obsolete?]
  -- Dockerfile and tests/run_tests.bash
  -- etc.
- Change '## Note: ...' to '# Note: ...' unless comments applies to
  another commemtn (e.g., TODO or OLD).
  -- Temporary comments use '##' rather than '#' to make this stick out
     more (e.g., to facilitate cleanup).
