ARROW-14146: [Dev] Update merge script to specify python3 in shebang line
ARROW-14146: [Dev] Update merge script to specify python3 in shebang line

This updates the shebang at the top of `dev/merge_arrow_pr.py` from `python` to `python3`

Closes #11247 from ianmcook/ARROW-14146

Authored-by: Ian Cook <14f1174bd060b5449094f39515ddcb18279f5391@gmail.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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