All __future__ imports (unicode_literals, print_function) are already
mandatory in py3. Also remove an outdated py2 comment in
presubmit_canned_checks.py
Bug: 1475402
Change-Id: I27cf6a8268f6dd1081f22af782c4c29a975376ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4867135
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>