factory: Import xrange from six package

`xrange()` is deprecated and renamed to `range()` in Python3.
However, `range()` in Python2 performs differently than `xrange()`,
so this CL replaces the build-in `xrange()` by `six.xrange()` to
make the function calls compatible in both versions.

BUG=chromium:999876
TEST=make test
TEST=umpire test
TEST=make lint with key enable=xrange-builtin on

Change-Id: Ib7e16078b2c6eda3029634ed7be104a3c156ab71
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/1868792
Reviewed-by: Yong Hong <yhong@chromium.org>
Commit-Queue: Yilin Yang <kerker@chromium.org>
Tested-by: Yilin Yang <kerker@chromium.org>
Auto-Submit: Yilin Yang <kerker@chromium.org>
diff --git a/py/toolkit/installer.py b/py/toolkit/installer.py
index 54e74a6..f39a0c3 100755
--- a/py/toolkit/installer.py
+++ b/py/toolkit/installer.py
@@ -24,6 +24,7 @@
 import time
 
 from six.moves import input
+from six.moves import xrange
 
 import factory_common  # pylint: disable=unused-import
 from cros.factory.test.env import paths