Switch to Github Actions

[goerz]

Following their aquisition by Idera, as of December 2020, Travis has closed travis-ci.org and reneged on their promise to support open source projects on travis-ci.com. Thus, Travis must be considered dead for open source development (and their paid tiers are prohibitively expensive).

The only realistic alternative is Github Actions. Thus we'll have to set up workflow files for automatic testing and generating the documentation. As a plus, Github Actions also support Windows and macOS, so we can replace AppVeyor at the same time.