autoninja.py uses subprocess.run which requires Python 3 and used
capture_output which requires Python 3.6. One user reported this as an
issue and it turns out that it is easy to avoid by using subprocess.NULL
which then means that Python versions back to 3.3 are supported.

Bug: 868590, b/174673874
Change-Id: Ife5e186d9c54747d35ff989dc2afadba5b9a57f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2630525
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>