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/loop_unittest.py b/scripts/sysmon/loop_unittest.py
index ab8001d..5c7bb2a 100644
--- a/scripts/sysmon/loop_unittest.py
+++ b/scripts/sysmon/loop_unittest.py
@@ -11,6 +11,7 @@
 from __future__ import print_function
 
 import contextlib
+import sys
 
 import mock
 
@@ -18,6 +19,9 @@
 from chromite.scripts.sysmon import loop
 
 
+assert sys.version_info >= (3, 6), 'This module requires Python 3.6+'
+
+
 class _MockTime(object):
   """Mock time and sleep.