When presubmit checks create warnings that should raise a prompt
whether or not to continue, the output wasn't being flushed
properly under python3.

Bug: 1213316
Change-Id: Ibb4b4d965d49f0cd65ccc0737282e5f5b5f58004
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2921323Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>