merge_jars.py: Make the code work on Python 3
merge_jars.py: Make the code work on Python 3

So far the gerrit build tool chain works as expected on Python 3. The
only problems that we are running into in merge_jars.py:

* ZipFile.read is returning byte and not string
* dict.iteritems() was removed in Python 3

Luckily, those problems can be easily rectified and code can be adjusted
to be compatible with both Python version 2 and 3.

Bug: Issue 7002
Change-Id: I487253b0e2e6fe4428fe60d57879424f85a198a0

WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,gerrit-review/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,WANdisco/gerrit,WANdisco/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,WANdisco/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit