global: start using black formatter

[mvidalgarcia]

https://black.readthedocs.io

Black is opinionated so you don’t have to be.

Motivation

• Have a deterministic way of formatting our code so we don't have to worry about it.
• Have a mechanism to auto format the code (like Prettier) so we save coding time.
• Use a formatter used by well-known projects and companies.

Things to take into account

• Start with small components to cause less impact (e.g. r-w-e-serial).
• Adapt tests to use something like black --check.
• Verify compatibility with isort.
• "It requires Python 3.6.0+ to run but you can reformat Python 2 code with it, too." Check that doesn't break python2-compatible code (e.g. reana-client). Explore --target-version flag:

   -t, --target-version [py27|py33|py34|py35|py36|py37|py38]
                        Python versions that should be supported by
                        Black's output. [default: per-file auto-detection]

• Add reana-team as author of bulk formatting changes.
• Consider adding a black badge to the README.
• Remove --pep8 from pytest.ini files as it causes conflicts with black.

Useful information

• Editor integration
• VSCode black extension has been deprecated and now it's part of the official Python extension which is a good sign for the future of the project.
• VSCode format on save

[mvidalgarcia]

All components migrated to Black 🎉