fix: Correct flake8 config

- Add exclude pattern for flake8 when running with pre-commit as it does
  not fully honor the exclude paths.
- Cleanup exclude paths for flake8 in tox.ini
- Re-enable line length check and correct three affected files

.pre-commit-config.yaml

@@ -14,6 +14,7 @@ repos:
     rev: 6.0.0
     hooks:
       - id: flake8
+        exclude: ^(.tox|env|build|dist|help|qt/dg_rc.py|pkg).*
   - repo: https://github.com/alessandrojcm/commitlint-pre-commit-hook
     rev: v9.3.0
     hooks: