The warning that a PRESUBMIT.py file was still being run under Python 2
was apparently too polite as one was just added. This change adds
instructions on how to prevent this, which should make it clear that
the behavior is not desirable.

The warning was added in crrev.com/c/3826917.

Bug: 1313804
Change-Id: I81a6ea45907b5ea00e7d945c3131a329708cc377
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3878875
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>