[BEAM-1251] Change unicode --> six.text_type for Python 3 - again

[cclauss]

unicode was removed in Python 3 because all str are Unicode so this PR is changing one instance of unicode to six.text_type.

DESCRIPTION HERE

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[cclauss]

@aaltay Can you please help me understand where the test failure is in https://builds.apache.org/job/beam_PreCommit_Python_MavenInstall/3557/org.apache.beam$beam-sdks-python/console ?

[aaltay]

@cclauss I think there was a Jenkins wide issue and nothing specific to this test was wrong. I could not find anything in the logs. And when I compare with the tests before and after, I noticed additional fields added (" Git Build Data") to the Jenkins UI for the tests ran after. I also checked other test suites, all seems to have failed tests at about same time. (cc: @alanmyrvold in case Alan knows something about this.)

Please retry running the test.

[holdenk]

So I did a live code review of an Apache Spark PR today, would you @cclauss be OK with me doing a live review of this (or one of your other PRs)? Not a committer on this project but I also want to show folks how they can help review in projects where they aren't committers.

If not no worries.

[cclauss]

Sure... No problems.

[aaltay]

LGTM.

cc: @udim