Enhancement/revise ci jobs

Daniel Scheffler requested to merge enhancement/revise_CI_jobs into master

I revised some continuous integration jobs that GitLab automatically runs as soon as somebody uploads new code.

  • new CI job for style checking
  • higher robustness of test_enpt_install job
  • updated versions of dependencies
  • updated test runner
  • job artifacts can now always be downloaded from CI pipelines page

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