ZOOKEEPER-3771: Update zk-merge-pr script to Python3
ZOOKEEPER-3771: Update zk-merge-pr script to Python3

eolivelli generally I use `2to3` util and check the codepath that I can arrive, manually fix some lines.

But it seems we can verify this patch totally when merging this patch :)

Author: tison <wander4096@gmail.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>, Norbert Kalmar <nkalmar@apache.org>

Closes #1295 from TisonKun/patch-1

arshadmohammad/zookeeper,arshadmohammad/zookeeper,maoling/zookeeper,apache/zookeeper,apache/zookeeper,maoling/zookeeper,apache/zookeeper,apache/zookeeper,maoling/zookeeper,hanm/zookeeper,arshadmohammad/zookeeper,arshadmohammad/zookeeper,arshadmohammad/zookeeper,hanm/zookeeper,maoling/zookeeper,hanm/zookeeper,maoling/zookeeper,arshadmohammad/zookeeper,apache/zookeeper,hanm/zookeeper,hanm/zookeeper,maoling/zookeeper,hanm/zookeeper,arshadmohammad/zookeeper,hanm/zookeeper,maoling/zookeeper,maoling/zookeeper,hanm/zookeeper,apache/zookeeper,arshadmohammad/zookeeper,apache/zookeeper,apache/zookeeper