Use black to format the code

[Guts]

What

black is code formatter for Python. It's presented as uncompromising as it applies most of choices without customization option in order to avoid endless discussions about the best line length, the right character for quotation marks, etc.

Why

• In multi-voices projects like this one, it avoids the different styles inherent to each contributor's tastes
• it makes the code base much more consistent and easy reading
• it's easy to use and integrate (IDEs...)
• it's supported by the PSF
• it automatically reduces a lot every linter warning (pylint, pycodestyle, etc.)

How

• [x] apply black to full source code base
• [ ] add it to contribution guidelines in style section
• [x] use pre-commit to auto-blacking code before to commit

[Guts]

Added to the contribution guidelines: https://github.com/QGIS-Contribution/QGIS-ResourceSharing/blob/master/CONTRIBUTING.md