cros_extract_deps/cros_workon: convert to python3 only

BUG=chromium:997354
TEST=CQ passes

Change-Id: I245f67c1b6319f8b4bd2dd13fa66c30743450247
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2111896
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_workon.py b/scripts/cros_workon.py
index 05f6d65..87fd961 100644
--- a/scripts/cros_workon.py
+++ b/scripts/cros_workon.py
@@ -13,12 +13,17 @@
 
 from __future__ import print_function
 
+import sys
+
 from chromite.lib import commandline
 from chromite.lib import cros_build_lib
 from chromite.lib import terminal
 from chromite.lib import workon_helper
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 def GetParser():
   """Get a CLI parser."""
   parser = commandline.ArgumentParser(description=__doc__)