STY: clean up `flake8` configuration

[fkuehlein]

• remove unnecessary extend-ignore
• correct per-file-ignores
• avoid single case of unused variable in utils.navigator

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 68.71%. Comparing base (ba83d3e) to head (d072998). Report is 5 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #233 +/- ## ========================================== + Coverage 68.63% 68.71% +0.08% ========================================== Files 44 44 Lines 6138 6138 ========================================== + Hits 4213 4218 +5 + Misses 1925 1920 -5 ```

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