[swarming] Use luci-go swarming in SwarmingClient.task_stdout()
[swarming] Use luci-go swarming in SwarmingClient.task_stdout()

Bug: 1135017
Change-Id: I1fc1e42447cc4f5dd0e85bbaa8b99742abede5f8
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3246728
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py