cli deploy/flash: convert to python3 only

All users of these modules have migrated and appear to be stable.

BUG=chromium:997354
TEST=CQ passes

Change-Id: I580f5532787d647d797e8c94157441d1c1410536
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2111901
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/cli/deploy_unittest.py b/cli/deploy_unittest.py
index 346f033..f96772f 100644
--- a/cli/deploy_unittest.py
+++ b/cli/deploy_unittest.py
@@ -23,6 +23,9 @@
               cros_test_lib.pytestmark_requires_portage]
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 if cros_build_lib.IsInsideChroot():
   import portage  # pylint: disable=import-error