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ARROW-13425: [Archery] Avoid importing PyArrow indirectly

Pandas will try to import PyArrow if seemingly available.
However, the currently installed PyArrow may not be compatible with the last compiled Arrow C++
(e.g. when using `archery benchmark diff ...`).

Closes #10769 from pitrou/ARROW-13425-archery-import-pyarrow

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>