Declare ruff as a runtime dependency in pyproject.toml

[albertvillanova]

This PR declares ruff as a runtime dependency in pyproject.toml, so that it is actually installed with the package.

Follow-up to:

• Add ruff to the list of dependencies #785

Motivation

doc-builder style invokes ruff unconditionally through subprocess.run(["ruff", ...]) when formatting code examples, so ruff must be present in the environment.

#785 added ruff to install_requires in setup.py, but that file is no longer used to build the package metadata: since #636, pyproject.toml declares a complete [project] table, which setuptools treats as authoritative. As a result, #785 had no effect.

This can be confirmed by inspecting the metadata of an install from the merge commit of #785:

$ pip install "git+https://github.com/huggingface/doc-builder@ffa5997"
$ python -c "from importlib.metadata import metadata; print(metadata('hf-doc-builder').get_all('Requires-Dist'))"
GitPython
...
httpx
ruff>=0.1.0; extra == "quality"

ruff only appears under the quality, all and dev extras, never as a core dependency. So a plain install still fails when styling a code example:

ValueError: Some code examples can't be interpreted by ruff, which means they aren't regular python:
Error message: [Errno 2] No such file or directory: 'ruff'

Note that this error only surfaces for docstrings or mdx files that contain code examples, and it is easily masked locally when ruff happens to be on PATH from an active virtual environment.

See the related downstream issue in trl:

• CI fails for code quality check: No such file or directory: 'ruff' trl#5633

Changes

• Add ruff to [project].dependencies in pyproject.toml

Note

setup.py is now unused for packaging metadata and has drifted: its install_requires still lists black and its extras["quality"] still pins black~=22.0, isort and flake8, all replaced by ruff in #636, while its extras["testing"] still lists requests, replaced by httpx in #754. Removing it would avoid similar confusion in the future. Left out of this PR to keep the change minimal.

[albertvillanova]

See CI becomes red again in trl: https://github.com/huggingface/trl/actions/runs/32150777426/job/95755816718