Change the source repository for the `black` hook

[mcdonnnj]

🗣 Description

This pull request changes the source repository we use for the black hook from psf/black to psf/black-pre-commit-mirror.

[!NOTE] This PR is built on top of #142 to hopefully improve merge queue success.

💭 Motivation and context

Using this repository will use the version of black available from PyPI which includes mypyc generated wheels. If one of these wheels is available (and used) it can offer significant performance gains vs. the standard Python package.

🧪 Testing

Automated tests pass.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All new and existing tests pass.