virtualenv_wrapper: convert all users to python3 only

BUG=chromium:997354
TEST=CQ passes

Change-Id: I0f93f61e56aa84efbab0386f1151bebd7f2f4f63
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2063224
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/sysmon/net_metrics_unittest.py b/scripts/sysmon/net_metrics_unittest.py
index 86d5a53..2489ff3 100644
--- a/scripts/sysmon/net_metrics_unittest.py
+++ b/scripts/sysmon/net_metrics_unittest.py
@@ -11,6 +11,7 @@
 from __future__ import print_function
 
 import socket
+import sys
 
 import mock
 import psutil  # pylint: disable=import-error
@@ -18,6 +19,10 @@
 from chromite.lib import cros_test_lib
 from chromite.scripts.sysmon import net_metrics
 
+
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 snetio = psutil._common.snetio
 snicstats = psutil._common.snicstats
 snic = getattr(psutil._common, 'snic', None)