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ARROW-8750: [Python] Correctly default to lz4 compression for Feather V2 in Python

This was the intention but I had not implemented it correctly. It's now tested to be so

Closes #7150 from wesm/ARROW-8750

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>