[client]Update utils/tools.py to make it python3 compatible.

It was missing in crrev.com/c/1637369.

Change-Id: I2b07fe7ca99ee82ab044a6dae3b2d9b20314a7f3
Bug: 942720
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1661146
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Mirrored-From: https://chromium.googlesource.com/infra/luci/luci-py
Cr-Mirrored-Commit: f8ab799f09a8152e3d276ba85f394e90d0fdc079
diff --git a/utils/tools.py b/utils/tools.py
index 377a61c..3b2a790 100644
--- a/utils/tools.py
+++ b/utils/tools.py
@@ -5,7 +5,6 @@
 """Various utility functions and classes not specific to any single area."""
 
 import atexit
-import cStringIO
 import functools
 import json
 import logging
@@ -18,6 +17,10 @@
 import utils
 from . import zip_package
 
+if sys.version_info.major == 2:
+  import cStringIO
+else:
+  import io as cStringIO
 
 # Path to (possibly extracted from zip) cacert.pem bundle file.
 # See get_cacerts_bundle().