client: fix SyntaxWarning
client: fix SyntaxWarning

This fix warning in python3.
/usr/local/google/home/tikuta/chrome_infra/infra/luci/client/utils/subprocess42.py:762: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if data or to is 0:

Bug: 1010816
Change-Id: Ia53e47ffb1d3e04ffd05b719c9af2df9c28e9fd5
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1875976
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Commit-Queue: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>

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