gm.py has been careful to only use the "pty" module's functionality
on Linux, but as it turns out, the module is *so* strongly specific
to Linux that even importing it fails on Windows. Making the import
of "pty" conditional makes gm.py work on Windows.

Change-Id: I0c1fb8a9a0299fde50e252337551d9395039f14d
Reviewed-on: https://chromium-review.googlesource.com/c/1382738Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58335}