Migrate to GitHub Actions #26

This project is currently still configured to use Travis CI, but is no longer hooked up to it. The preferred flow now uses GitHub Actions—an example can be seen in code companion for Publishing Python Packages.

Outcome

• Travis CI configuration is removed
• GitHub Actions provides feedback on each pull request about the unit tests and linting
• GitHub Actions triggers a PyPI release and creates a GitHub release with artifacts when an appropriate version tag is created (may require secrets to be set, which can be coordinated later)