chore: add preview option to black pre-commit hook and update README

[Mustaballer]

What kind of change does this PR introduce?

Add --preview flag to python black pre-commit hook, and update README.md on black pre-commit hook.

Summary

We want to use future python black formatting as mentioned here.

Checklist

• My code follows the style guidelines of OpenAdapt
• I have performed a self-review of my code
• If applicable, I have added tests to prove my fix is functional/effective
• I have linted my code locally prior to submission
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. README.md, requirements.txt)
• New and existing unit tests pass locally with my changes

How can your code be run and tested?

Run pre-commit install to update pre-commit hooks and test when staging files that need formatting changes.

[Mustaballer]

Removed because it is already specified in pyproject.toml