cros_setup_toolchains: convert to python3 only

All users of this module are python3 only already.

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

Change-Id: Iddad26d23c2530339c762558d8926e991ed1f7e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2190157
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Michael Mortensen <mmortensen@google.com>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index fe0c280..d96a4a1 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -14,6 +14,7 @@
 import os
 import re
 import shutil
+import sys
 
 from chromite.lib import constants
 from chromite.lib import commandline
@@ -35,6 +36,9 @@
   import portage
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 EMERGE_CMD = os.path.join(constants.CHROMITE_BIN_DIR, 'parallel_emerge')
 PACKAGE_STABLE = '[stable]'