Re-establish Python 3.7 compatibility

[zklaus]

Description

When adding provenance to the diagnostic script, I made use of the walrus operator (:=) which was introduced in Python 3.8.
Since we seem to still be supporting Python 3.7, this is reformulated here with a compatible version. Use of pre-commit hooks also triggered a few reformats and the fixing of some typos.

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New or updated recipe/diagnostic

• 🧪 Recipe runs successfully
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• 🧪 Figure(s) and data look as expected from literature
• 🛠 Provenance information has been added

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[axel-lauer]

I just tested the recipe once more. Works fine, provenance output also looks fine.