adjust ETL generator

[oxe-i]

I checked all the easy exercises, they are all working fine. I think this was the only test file in need of a readability improvement. Maybe high-scores too, but I think it is good enough as it is.

Tomorrow, or maybe Thursday, I'll check the medium exercises. The hard exercises I'll leave to after launching.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[no important files changed]

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