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Add python3 compatibility to policy_testserver.py. Python 2
support is maintained for autotest.

TEST=tast run <ip> policy.AllowDinosaurEasterEgg with python 2 and 3

Bug: chromium:1041448
Change-Id: If8e7d5163d9bc753c17f5dae8731f64652810675
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2206636Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Victor-Gabriel Savu <vsavu@google.com>
Cr-Commit-Position: refs/heads/master@{#770507}