Running on Python 3 should be a fairly common situation at this
point, so it doesn't seem like we need an explicit warning for it.

Change-Id: Id73ffb89f6da6e7268fd2e5963cf99175a47b5b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2284547Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>