[Telemetry] Replace six.ensure_str() with decode()
[Telemetry] Replace six.ensure_str() with decode()

ensure_str is not part of the six module in Chrome OS telemetry environment.

Verified in
https://ci.chromium.org/ui/p/chromeos/builders/test_runner/test_runner/b8820715836828541297/overview.

b:222562164, b:206008069
Change-Id: I7d0c0fb7740bf34c65d23b10de9384eeb3828f8e
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/3499047
Reviewed-by: John Chen <334fbfbb4df7c78f091ea1b77c69ca6ac731a3f3@chromium.org>
Commit-Queue: Denis Nikitin <e7fe191ec6aaf0dbca7e14bd4d02d4bd4008ab8f@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult