Add ruff configuration to `pyproject.toml`.
This allows `ruff format` in the Spack repository to format all the files we care about,
with our line length of 99, the exceptions we already put in place, and excluding things
we don't auto-format, like vendored dependencies.
Right now it'll reformat 175 or so files, but only slightly, in places where `ruff` differs from
`black`. For the most part I like the ruff format decisions better than `black`, but none of
the changes seem too severe.
This does not change `spack style` -- I figure that can come later but this at least will
let people start playing with `ruff`.
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Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
71d1901831