[swarming] fix errors to import Python2 specific Mac OS modules
[swarming] fix errors to import Python2 specific Mac OS modules

'Carbon' and 'MacOS' don't exist in Python3.
This makes the test runner fail with import errors when running in Python3.

Bug: 1100214
Change-Id: Id42d479aa3db72ed813229d56ab9f92eb073642e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2275157
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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