Deprecation notice: v3 of the artifact actions

[gatoniel]

The upload-artifact and download-artifact actions are currently on version 3. They have to be updated to v4 by 30th of Nov. 2024, otherwise they will no longer be executed on github. See https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/

There are some breaking changes that affect the current workflows:

1. As hidden files .coverage.* are uploaded this has to be explicitly marked with the include-hidden-files: true directive
2. In one workflow, files cannot be uploaded to the same artifact name in multiple jobs. The artifacts have to have unique names within a workflow. This can be solved by using the name coverage-data-${{ matrix.python }}-${{ matrix.os }}. Then, in the download-artifact the directives pattern: coverage-data-* and merge-multiple: true can be used.