autotest_quickmerge: migrate to python3 only

BUG=chromium:997354
TEST=CQ passes

Change-Id: Idda7389100294f50dcd99777e1bf6d741523c55b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2068026
Reviewed-by: Kevin Shelton <kmshelton@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/autotest_quickmerge_unittest.py b/scripts/autotest_quickmerge_unittest.py
index 73697fa..b519771 100644
--- a/scripts/autotest_quickmerge_unittest.py
+++ b/scripts/autotest_quickmerge_unittest.py
@@ -7,12 +7,17 @@
 
 from __future__ import print_function
 
+import sys
+
 import mock
 
 from chromite.lib import cros_test_lib
 from chromite.scripts import autotest_quickmerge
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 RSYNC_TEST_OUTPUT = """.d..t...... ./
 >f..t...... touched file with spaces
 >f..t...... touched_file