Write black diff on errors

[max-sixty]

• Passes isort . && black . && mypy . && flake8
• User visible changes (including notable bug fixes) are documented in whats-new.rst

Inspired by @Illviljan in #4750

[keewis]

I would do that for isort, too, but otherwise looks good to me.

Be aware that #4730 replaces the current style check CI with a pre-commit CI, which automatically prints all changes.

[max-sixty]

Oh nice, I hadn't seen that. That's much better.

I'll merge this regardless but we can remove along with the rest of Azure when that merges — hopefully this will have a limited lifespan 😄