Fix trigger.py to run mb.py with python3 Bug: angleproject:5707 Change-Id: Ib0e8db962e89c536f92d8bfef8f52a4600e9bc35 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3645125 Reviewed-by: Yuxin Hu <yuxinhu@google.com> Auto-Submit: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Yuxin Hu <yuxinhu@google.com>