Add flake8 step in the build pipeline

[mgoerens]

No description provided.

[mgoerens]

Note that this should be merge after #399

[komish]

Does this step fail when the code doesn't match style guidelines? Just want to confirm as applying them here won't do anything, and we want the step to fail so the contributor can make their fixes and repush.

[mgoerens]

Yes, the step fails if flake8 complains. See for instance: https://github.com/mgoerens/my-chart-verifier/actions/runs/5750215106/job/15586524282

Actually, flake8 only ever emits errors, it doesn't edit the code. I use black for that.

[komish]

@mgoerens okay perfect.

One more thing, can we either

1. Use a GitHub action for Flake8 (e.g. https://github.com/marketplace/actions/flake8-action, but any one you like will be fine)
2. Install Flake8 into its own venv?

Just looking to separate it a bit to avoid any kind of dependency conflict in ve1.

[mgoerens]

I now installed flake8 in a dedicated venv.

[komish]

/lgtm