MNT Remove unused extra_requires from pyproject.toml - #34544

Reference Issues/PRs

Closes #34537

What does this implement/fix? Explain your changes.

Removes the [project.optional-dependencies] extras (build, install, benchmark, docs, examples, tests, maintenance) from pyproject.toml. See why in the issue.

Also:

• Removed the tag_to_packages inverse mapping in sklearn/_min_dependencies.py. It was already dead code since setup.py's extras_require (its only consumer) was removed in BLD Remove support for setuptools build #29400.
• Renamed sklearn/tests/test_min_dependencies_readme.py -> test_min_dependencies_consistency.py because it tests consistency of sklearn/_min_dependencies.py against the doc and the pyproject.toml.
• Updated it to drop checks on the now-removed project.optional-dependencies.* sections

AI usage disclosure

I used AI assistance for:

• Code generation
• Test generation
• Research and understanding

Follow-up question

Tags in dependent_packages are now used this way:

• build & install tags to cross-check pyproject.toml
• others tags are only used to render the minimum-dependency table in the docs. For me the table it generates s a bit confusing for end-users. It's more contributor oriented but it's in the installing-the-latest-release section... Maybe we could revisit that?