Polish nox setup

[jodal]

• Type noxfile
• Define all nox Python versions once

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (059bc69) 100.00% compared to head (2952b7b) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #185 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 18 18 Lines 912 912 Branches 146 146 ========================================= Hits 912 912 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stein+Magnus+Jodal)

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