[SPARK-28586][INFRA] Make merge-spark-pr script compatible with Python 3
[SPARK-28586][INFRA] Make merge-spark-pr script compatible with Python 3

## What changes were proposed in this pull request?

This PR proposes to make `merge_spark_pr.py` script Python 3 compatible.

## How was this patch tested?

Manually tested against my forked remote with the PR and JIRA below:

https://github.com/apache/spark/pull/25321
https://github.com/apache/spark/pull/25286
https://issues.apache.org/jira/browse/SPARK-28153

Closes #25322 from HyukjinKwon/merge-script.

Authored-by: HyukjinKwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
Signed-off-by: Dongjoon Hyun <9b02cb7e37158797a8e22c559f7c5f5ecd28ea57@apple.com>

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