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ARROW-9385: [Python] Fix JPype tests and JVM buffer lifetime

Closes #7753 from rymurr/ARROW-9385

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Ryan Murray <rymurr@dremio.com>
Co-authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>