python: switch to logging.warning

No change to logic. logging.warn() is deprecated. Switch to
logging.warning().

This CL was uploaded by git cl split.

R=pasko@chromium.org

Bug: 40701320
Change-Id: Iccb0a380e60123313d8449ef580dbec3bffdd533
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402725


Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280155}