systrace: represent git hash as string in Python3
systrace: represent git hash as string in Python3

In Python3 subprocess.check_output() return value will be represented
as bytes instead of string. So manually convert this.

This should no affect general tool functionality but will impact messages
printed to the console when run under python3.

Change-Id: Id4be5cadccf238776b15c656f37efe668982e490
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/3781217
Commit-Queue: John Reck <71b30919c768f293772415a2a3deb09223257fb5@google.com>
Reviewed-by: John Chen <334fbfbb4df7c78f091ea1b77c69ca6ac731a3f3@chromium.org>
Reviewed-by: John Reck <71b30919c768f293772415a2a3deb09223257fb5@google.com>

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