[CFP-506] Add pr backend linter

[w1stler]

Story / Bug id:

507

Description:

Added PR linter for backend

[arturtamborski]

This one is now looking pretty good, just missing a few details (see suggestion).

[Deelite34]

I had a look at this PR.
PR itself is fine, however as I found out, the issue with github action linting failing lies within lint backend action in actions repository.
It seems that the latest version of black is being used there, despite 22.12.0 version being intended to be used, in result causing this problem.
To fix this inconsistency and use 22.12.0 properly, set version parameter in actions repo, then I think new release needs to be created there, and used in here in this github action file. I've tested and got linting action to pass on a fork.
source

    - uses: psf/[email protected]
      with:
        version: "22.12.0"
        src: backend