Scripts: Update bootstrap.py to python3 Bug: angleproject:5829 Change-Id: I564fadee92b6ff301c055cf95a163ffb3c838685 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2810338 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>