[client] cast kvs filepath as six.text_type
[client] cast kvs filepath as six.text_type

This is fix for http://shortn/_mEkfrJgHi8

Bug: 1152663
Change-Id: I490eb03d03667f0ca65347d059250bcab1092a1a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2569299
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