cros_test/cros_vm: convert to python3 only

BUG=chromium:997354
TEST=CQ passes

Change-Id: I5de603a5ce35e6c6dca07c0b993b00bb9583ec61
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2068817
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
diff --git a/cli/command_vm_test.py b/cli/command_vm_test.py
index add963c..0013db8 100644
--- a/cli/command_vm_test.py
+++ b/cli/command_vm_test.py
@@ -11,6 +11,8 @@
 
 from __future__ import print_function
 
+import sys
+
 from chromite.cli import deploy
 from chromite.lib import constants
 from chromite.lib import cros_build_lib
@@ -20,6 +22,9 @@
 from chromite.utils import outcap
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 class Error(Exception):
   """Base exception for CLI command VM tests."""