Feature/code style automation

[diptorupd]

Various changes to code style tooling:

1. flake8, isort jobs added to check for Python code style for new PRs against master, gold/2021, release branches.
2. black max line width changed to 80 chars.
3. flake8 and isort configs added.
4. pre-commit hooks added to make it possible to run all these checks prior to pushing code.
5. CONTRIBUTION.md updated.

[oleksandr-pavlyk]

It is imperative that source code passes all the linter checks before this effort is merged.

My attempt to take this for a ride revealed black auto-formats some code that flake8 flags, example of it being

__all__ = (a
   + b 
   + c
   + d )

Flake8 runs on Cython files, and flags legal Cython extensions of Python as syntactic errors.

[diptorupd]

It is imperative that source code passes all the linter checks before this effort is merged.

My attempt to take this for a ride revealed black auto-formats some code that flake8 flags, example of it being

__all__ = (a
   + b 
   + c
   + d )

Flake8 runs on Cython files, and flags legal Cython extensions of Python as syntactic errors.

The black docs point out to this possibility and provide a flake config option. I have added the exception to the .flake8 file.

[PokhodenkoSA]

@diptorupd I know that muting warnings fixes CI. I would prefer the following way: run checks locally. Find most danger warnings. Fix in separate PRs and the apply this PR.

[PokhodenkoSA]

Fix in separate PRs and the apply this PR.

It will split CI changes from fixes and code improvements.

[diptorupd]

It will split CI changes from fixes and code improvements.

Yes, #416 has the fixes. I just rebased #416 on top of this PR to see the results. Ultimately, I will merge #416 separately and this PR will have only the code improvement checks here.