Remove uses of logging.warn.

logging.warn is deprecated and we should always use logging.warning
(http://bugs.python.org/issue13235).

BUG=None
TEST=Unittests.

Change-Id: I8ca32a6f0d2d96baecb4a1e3f70d2273d1894a06
Reviewed-on: https://chromium-review.googlesource.com/272445
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Trybot-Ready: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Steve Fung <stevefung@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/scripts/loman.py b/scripts/loman.py
index 5de8c2f..d068c4e 100644
--- a/scripts/loman.py
+++ b/scripts/loman.py
@@ -187,7 +187,7 @@
       _UpgradeMinilayout(options)
     return 0
   elif upgrade_required:
-    logging.warn(
+    logging.warning(
         "Your repository checkout is using the old minilayout.xml workflow; "
         "auto-upgrading it.")
     cros_build_lib.RunCommand(