Consider switching CI/CD to GitHub Actions

[Pedro Morales Maries]

Consider switching CI/CD pipelines to GH Actions.

Currently the project has:

• travis-ci: for Linux and MacOS (does not integrate out-of-the-box with the checks of PRs anymore)
• appveyor: for Windows

In theory both CI providers could be switched to GH Actions (AFAIK, GH Actions supports all the mentioned platforms).