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.py b/cli/deploy.py
index e25b576..bf42e15 100644
--- a/cli/deploy.py
+++ b/cli/deploy.py
@@ -17,6 +17,7 @@
 import functools
 import json
 import os
+import sys
 import tempfile
 
 from chromite.cli import command
@@ -34,6 +35,9 @@
     raise
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 _DEVICE_BASE_DIR = '/usr/local/tmp/cros-deploy'
 # This is defined in src/platform/dev/builder.py
 _STRIPPED_PACKAGES_DIR = 'stripped-packages'