apply black formatting and add CI run

What does the merge request do?

Apply the Black python formatter to the entire codebase, as discussed in yesterday's dev meeting. This is one giant commit, but good news, you will never see whitespace changes again :)

Also added a config for a git pre-commit hook and updated the CI to include a check for codestyle compliance.

Implementation notes

• The maximum line length in pylintrc has been changed to 88 characters, which is what Black uses (https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html).
• I haven't used any formatting on/off switches.

Summary for release notes

The code formatting of the extensions repository has been made consistent. This enables contributors to think less about formatting and more about what matters.

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history