Switch from pylint to flake8 for linting

[lonvia]

mypy is doing a superb job when it comes to detecting programming errors and buggy code. The additional issues that pylint is flagging are smaller style issues that are not without problems. The number of disabled checks has become quite long. And every version comes with new checks that breaks CI at regular intervals. At the same time pylint is severely lacking checks for PEP8 coding style, in particular, with respect to indention, white spaces and blank lines. flake8 has a much better support in that area.

This PR brings the code up to PEP8 standards and switches to flake8 for linting.

At the moment this only includes the core checks of flake8. There are a couple of interesting plugins like flake8-quote or flake8-return, which I might add in later PRs.