Black: Applied black formatting

[CSSFrancis]

Description of the change

Add black formatting to diffsims. With the added checking in the CI this is starting to make lots of PR's look confusing.

Progress of the PR

• [ ] Docstrings for all functions
• [ ] Unit tests with pytest for all lines
• [ ] Clean code style by running black

For reviewers

• [ ] The PR title is short, concise, and will make sense 1 year later.
• [ ] New functions are imported in corresponding __init__.py.
• [ ] New features, API changes, and deprecations are mentioned in the unreleased section in CHANGELOG.rst.
• [ ] Contributor(s) are listed correctly in credits in diffsims/release_info.py and in .zenodo.json.