replication_util: switch to python3 only

No one imports this, just runs it, so should be easy to convert.

BUG=chromium:997354
TEST=`./run_tests` passes

Change-Id: Iee67939aafbcbc33d7df5a810c6556887a85c649
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2052464
Reviewed-by: Andrew Lamb <andrewlamb@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/replication_util_unittest.py b/scripts/replication_util_unittest.py
index c09f45c..c6fca4e 100644
--- a/scripts/replication_util_unittest.py
+++ b/scripts/replication_util_unittest.py
@@ -9,6 +9,7 @@
 
 import json
 import os
+import sys
 
 from google.protobuf import json_format
 
@@ -20,6 +21,10 @@
 from chromite.lib import osutils
 from chromite.scripts import replication_util
 
+
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 D = cros_test_lib.Directory