Version 0.2.0 release

• Drop Python 2 support.
• Minor updates to package metadata.
• Update docs build, use readthedocs v2 configuration.
• Simplify package maintainence and tooling. tox removed. flake8 replaced for ruff. Testing now just ruff and pytest.
• Switch from setup.py to pyproject.toml with hatchling for builds and an /src layout.
• Remove Travis CI integration and configurations in favor of Github Actions.