Use Python 3 in trigger.py. Bug: angleproject:5707 Change-Id: I7b1281711133ef3a60257c602f1af1db7f3af85f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3284395 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>