Replace flake8 checks with Ruff and show the warning messages onh GH

[rdbende]

What we discussed in PM @geigi, except that I added Ruff instead, so it finishes much faster. Ruff also has a ton of rules built-in (https://github.com/astral-sh/ruff#rules), so we might need to adjust the --select later.

With the --output-format github it produces outputs in the Files tab of the PR like this:

After this, we can turn off the Codeclimate checks.

[rdbende]

I left --exit-zero there for now, so it won't make every PR fail. However it would be good to fix all those warnings it gives in a follow-up pull request, and remove this flag.

[geigi]

Awesome :) Ruff looks cool, didn't know it yet! I'll disable Codeclimate