Required Black and isort code formatters not in pyproject.toml

[dokterbob]

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Describe the bug The pre-commit hook calls black, but it's not mentioned as a (dev) dependency in pyproject.toml. They are explicitly mentioned in the contribution instructions.

Adding this to dev dependencies (rather than main deps) would facilitate running them on CI in addition to making dev easier and more consistent (together with #1190).

Observations There's currently a tests and a mypy group but not a dev group. There is no group containing these code formatters.

What is the rationale behind this, if there is any? Is there a use case when mypy is required but tests aren't, and vice versa? Are there use cases where black and/or isort are not required?

Suggestion A common practice is to have a single dev group with all dev-related requirements, used both for local dev and remote CI. This is conceptually easier.