Launch swarming on CT GCE instances with python3
Launch swarming on CT GCE instances with python3

Also fix code to download swarming_bot.zip

Bug: skia:12260
Change-Id: If01601605f072e2226be10e3ad03f79b86c6a328
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/436096
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot