Add diff output to black workflow step

[stkerr]

Description

This change adds the --diff flag when running Black. This will make it print out what it would expect to see. This makes it easier for developers to understand what they need to change for Black to be happy. This removes the need for users to worry about running a wrong version of Black locally that may be looking at different things than the canonical Black is looking at.

How has this been tested?

• Please describe the tests that you ran to verify your changes.
• Provide instructions so we can reproduce.
• Please also list any relevant details for your test configuration.

Checklist:

• Update our Hugo documentation following these guidelines.
• Update our tests following these guidelines.
• Make sure you are following our CONTRIBUTING guidelines.
• If a feature was added make sure to add it to the corresponding scripts file.

Others

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My code passes all the checks pylint, flake8, black, ... To speed up development you should run pre-commit install.
• New and existing unit tests pass locally with my changes. You can test this locally using pytest tests/....

[stkerr]

@DidierRLopes @jmaslek can you review & merge this PR? Thanks!

[DidierRLopes]

This is nice. I wish we had done this long ago 😓