MNT Remove setuptools dependency in our test suite

[lesteve]

This was noticed when working on Python 3.12 wheels in #27027. In Python 3.12 setuptools is not a core dependency venv (see this).

As noted in #27027 (comment), our test suite should not depend on setuptools.

This PR:

• remove test_tabs which is not useful since we are using black
• avoids importing sklearn._build_utils.pre_build_helpers (which is the only module depending on setuptools) to check its __all__ in test_import_all_consistency`

[github-actions]

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[thomasjpfan]

Does the act of using pkgutil.pkgutil.walk_packages to go through every submodule which includes _build_utils end up importing setuptools anyways?

XREF: #26992

[lesteve]

My understanding is that it will indeed try to import all the submodules (and hence try to import setuptools), but that ImportError is going to be ignored. By default (onerror=None), only ImportError is ignored. When specifying onerror, any kind of Exception is ignored:
https://github.com/python/cpython/blob/1f885df2a580360c5de69cc41191f3c6bfaaeb35/Lib/pkgutil.py#L76-L86

In other words, there is no hard dependency on setuptools by using pkgutil.walk_packages but this may still have side-effects as in #26992 if setuptools can be imported.

[ogrisel]

LGTM, thanks very much @lesteve.