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/flash_unittest.py b/cli/flash_unittest.py
index 8a0077f..aceea1d 100644
--- a/cli/flash_unittest.py
+++ b/cli/flash_unittest.py
@@ -8,6 +8,7 @@
 from __future__ import print_function
 
 import os
+import sys
 
 import mock
 
@@ -27,6 +28,9 @@
 from chromite.lib.paygen import paygen_stateful_payload_lib
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 class RemoteDeviceUpdaterMock(partial_mock.PartialCmdMock):
   """Mock out RemoteDeviceUpdater."""
   TARGET = 'chromite.lib.auto_updater.ChromiumOSUpdater'