fix linting and openpyxl

Summary

See #122 (closed): Do not use openpyxl in 3.1.3. Also some linting.

Focus

Nothing special

Test Environment

Reinstall dependencies, run unittests.

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• All automated tests pass
• Reference related issues
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Check List for the Reviewer

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• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
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For further good practices have a look at our review guidelines.