Move linting/code style checks to a separate test

[JackDyre]

I would be nice if the linting/code style tests were in their own test. It is kinda confusing when I make a change and all the tests fail, but it is only because I forgot to run the linting commands and the all of the tests passed.

[stephanebruckert]

We could split linting/testing into different workflows so it shows more clearly what's green or red. Also we could only run the linting step or workflow when the PR is not a draft? (actually a bad idea, someone might want to focus mostly on linting)

On your side you could also look at setting up flake8 in your code editor so it fixes code automatically.

[dieser-niko]

While we're at it, we might want to remove the line pip install -Iv enum34==1.1.6, as we don't support versions below 3.4 anyway.