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This PR removes the files for the appveyor and travis CI, since they were superseded by GitHub actions in #278.
Removing travis CI is fine I think, because travis is no longer providing a reasonable free CI service.
Appveyor still works and is even providing older versions of Visual Studio than GitHub actions. E.g. VS2015. Do we care about such old versions? If not, we could even consider to move the minimum Visual Studio version to 2017, which is supported in GitHub actions.
This PR depends on #297, which needs to be merged first.