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.py b/cli/flash.py
index 902b193..8fca57d 100644
--- a/cli/flash.py
+++ b/cli/flash.py
@@ -12,6 +12,7 @@
 import re
 import shutil
 import subprocess
+import sys
 import tempfile
 
 from chromite.lib import constants
@@ -32,6 +33,9 @@
 from chromite.lib.xbuddy import artifact_info
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 DEVSERVER_STATIC_DIR = path_util.FromChrootPath(
     os.path.join(constants.CHROOT_SOURCE_ROOT, 'devserver', 'static'))