CI/CD workflow now based on GitHub Actions

GitHub offers with GitHub Actions an environment for CI/CD workflows that is free of charge for open source projects. After Travis's service stopped in June 2021, see the blog post, it was natural to port the existing workflow from Travis to GitHub Actions. Since the core functionality of GitHub Actions is similar to that of Travis, it was easy to adapt the workflow description. GitHub Actions support the currently up-to-date Ubuntu 2020.04 LTS, the build matrix reflects therefore my current development environment

The workflow logic is identical to the Travis implementation, see the blog post for a brief description. For details see .github/workflows/ci.yml of commit db7556a4.