client: fix some isolateserver tests on python3
client: fix some isolateserver tests on python3

Bug: 1010816
Change-Id: I569cb5bf3c4cdbe91f6ce33d953f422c554d0186
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2286496
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