build: revert flake8 pin

[alexander-held]

Resolves #379: the pin flake8<6 introduced temporarily in #380 can be removed, given that a compatible flake8-import-order version now exists.

* remove temporary flake8 pin
* fix formatting issue with flake8 extend-ignore setting

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (7d7e557) compared to base (5e00b3d). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #384 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 23 23 Lines 2041 2041 Branches 324 324 ========================================= Hits 2041 2041 ``` 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=scikit-hep). 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=scikit-hep)

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