client: fix syntax warning in isolate_format.py
client: fix syntax warning in isolate_format.py

This fixes following warning in python3.
 SyntaxWarning: invalid escape sequence \)

Bug: 1010816
Change-Id: Ibfd08cc6e34a9f678dd6abef995322a4fa0191bd
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1878208
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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