client: make file_path_test.py python3 compatible
client: make file_path_test.py python3 compatible

Bug: 1010816
Change-Id: I77da6c90084610c2cb641289eb9b1ef2c5591ecb
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1874419
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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