ARROW-3329: [Python] Python tests for decimal to int and decimal to decimal casts
ARROW-3329: [Python] Python tests for decimal to int and decimal to decimal casts

Closes #6846 from JacekPliszka/ARROW-3329

Lead-authored-by: Jacek Pliszka <b7ce3da0c8cd063858fbbd4667cc2c658a558484@gmail.com>
Co-authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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