Restore automatic flake8 linting on travis

[Nabellaleen]

Due to #188 refactoring to migrate to tox, the automatic flake8 on travis is not triggered anymore

We should fix it for the 2.1.1 release

[github-actions]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related topics referencing this issue.