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.py b/scripts/autotest_quickmerge.py
index d26fa80..ef9d0a3 100644
--- a/scripts/autotest_quickmerge.py
+++ b/scripts/autotest_quickmerge.py
@@ -31,6 +31,9 @@
   import portage
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 INCLUDE_PATTERNS_FILENAME = 'autotest-quickmerge-includepatterns'
 AUTOTEST_PROJECT_NAME = 'chromiumos/third_party/autotest'
 AUTOTEST_EBUILD = 'chromeos-base/autotest'