This closes #695 by removing all references to TravisCI and adding a GitHub Actions workflow for testing and linting instead.

A few notes:

• Instead of tox, this workflow uses GitHub Actions’ native capabilities for testing across multiple Python versions. This seems more straightforward and is more easily extensible e.g. to test on different operating systems. I’ve kept tox everywhere else, for local testing. (Personally, I’d be tempted to remove it completely—but that’s better left to a separate PR.)
• It is also possible to set up a GitHub Actions workflow to automatically publish to PyPI. GitHub docs have a complete workflow template for that; for now, I’ve included a link to that in CONTRIBUTING.rsts “Deployment” section, but I could also add that workflow to the template directly, if you think that makes sense?