chore(flake8): add --show-source option to flake8

Hello dear contributors,

This is an auto generated modification to help you improve the code base of logilab. If this commit is bad don't hesitate to close this MR and report it here https://forge.extranet.logilab.fr/cubicweb/cube-doctor/-/issues.

Kind regards,

added To Review label

restored source branch topic/default/show-flake8-sources

enabled an automatic merge when the pipeline for 6fb35126ea53 succeeds

requested review from @alubert

merged