[WIP] Introduce code-quality checks to the repo (details TBD)

[devProdigy]

What this PR offers:

• check code-quality with flakehell (flake8 + plugins) and "black" on the CI before tests. In such way we'll eliminate waiting time to know that some space is missing.
  • I know that there's a lot of legacy stuff here and no code-analysers (except black) was run previously, so this checks run only on changed code. Old code could be "unified" with time.
• separate code-quality steps from tests
  • here's CI job that shows how the flow looks like and how caching with pythonLocation speeds up the process. If I missed any downsides/risks of using pythonLocation instead of pip cache - please let me know. We should fix cache limit range.
  • here's job that shows how CI will behave on quality-check error.

Motivation:

• unifies code is easier to read;
• code-checkers allow us not to spend code-review time on comments with formatting and styles discussions;
• etc.

P.S. Previous PR was closed due to some un-reverted issues :) Here's lates comment from @flikka about this idea. P.S.2. Feel free to add your opinion :)