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ARROW-13187: [Python] Avoid creating reference cycle when reading CSV file

Some Python versions have a bug where `signal.getsignal` creates a reference cycle holding execution frames alive (https://bugs.python.org/issue42248).

This would cause excessive lifetimes of the PyArrow table returned by `read_csv`.

Closes #10609 from pitrou/ARROW-13187-signal-refcycle

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