Code linting on build

[nolar]

Code should be automatically linted on every push, as part of the building process.

The coding guidelines should be some defaults of pylint, flake8, or both. One exception:

• Line length is 100 chars.

No linting scripts, no CLI options, just the standard CLI tool should already take all this into account — i.e. auto-detectable configs of these tools should be used (same as by the IDE).