client: run isolateserver_smoke_test in python3
client: run isolateserver_smoke_test in python3

This can already run in python3.

Bug: 1010816
Change-Id: I857f8c335eebd536533014e22ecd3edcbd8fbc57
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2279453
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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