build: temporarily pin flake8<6

[alexander-held]

flake8-import-order is incompatible with flake8 version 6.0.0 at the moment, see #379. Temporarily pin to avoid issues and then remove pin once everything is compatible again.

* temporarily pin flake8<6 due to incompatibility with flake8-import-order

[codecov[bot]]

Codecov Report

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

Coverage data is based on head (051cbdb) compared to base (4181499). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #380 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 23 23 Lines 2020 2020 Branches 319 319 ========================================= Hits 2020 2020 ``` 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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