venvlib: avoid 3rd party imports

We use this to bootstrap the virtualenv, so we can't rely on any
3rd party imports in the system.

BUG=chromium:1052692
TEST=CQ passes

Change-Id: I8cc152205a9c756734ca5d9a06e66ce7d3042679
diff --git a/venv/cros_venv/venvlib.py b/venv/cros_venv/venvlib.py
index a098e14..a8034c3 100644
--- a/venv/cros_venv/venvlib.py
+++ b/venv/cros_venv/venvlib.py
@@ -23,8 +23,6 @@
 import tempfile
 import warnings
 
-import six
-
 from cros_venv import constants
 from cros_venv import flock
 
@@ -243,7 +241,7 @@
     try:
         _log_check_call(command, logfile=logfile, env=env)
     except OSError as e:
-        six.reraise(VirtualenvMissingError(e), None, sys.exc_info()[2])
+        raise VirtualenvMissingError(e)
 
 
 class VirtualenvMissingError(Exception):