Migrating project metadata from setup.cfg to pyproject.toml triggers black changes

[WilliamJamieson]

Describe the bug

astropy is in the process of migrating its project metadata from the setup.cfg file to the pyproject.toml file in astropy/astropy#15247, as part of this process it triggered black making the changes in astropy/astropy#15254.

black making these changes was surprising as astropy has always configured black via the pyproject.toml file and no changes to the black configuration were made as part of astropy/astropy#15247. This seems like a bug if no changes to python files or the black configuration triggers black to make changes.

To Reproduce

Apply the changes from 3b9ceb1 (migration of project metatdata from setup.cfg to pyproject.toml) 7bc6005 (current head of astropy development) and then run black.

Expected behavior

No changes should be made by black.

Current behavior

The changes in 667bfd4 are made by black as part of the astropy pre-commit.

Environment

• Black's version: 23.7.0
• pre-commit.ci: (configured by https://github.com/astropy/astropy/blob/7bc600506dad3c1ce697a68fcbbc395256a438a4/.pre-commit-config.yaml#L72-L75)

Additional context

I am not sure if this is an actual bug or just some flaw in the proposed migration of astropy's setup.cfg. In either case, it is surprising that a change unrelated to black or any python file triggers changes to be made by black.

[JelleZijlstra]

Probably due to #3219? You added requires_python= >=3.9 and I believe that triggers Black to format only in 3.9+ mode.

[WilliamJamieson]

astropy's current configuration specifies python_requires = >=3.9 in the setup.cfg. This should do the same thing?

[JelleZijlstra]

No, Black only looks at pyproject.toml, not at the equivalent config in setup.cfg.

[WilliamJamieson]

In that case this is likely the cause of the issue. Thanks for your help.