Force Python 3 in presubmit

BUG=dawn:1015

Change-Id: I80d5afcfe3a7fc5c3c4c79d8c423ddf9578712e0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/58960
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

PRESUBMIT.py

@@ -16,6 +16,8 @@ import os
 import platform
 import subprocess
 
+USE_PYTHON3 = True
+
 
 def _DoCommonChecks(input_api, output_api):
     results = []

infra/config/PRESUBMIT.py

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+USE_PYTHON3 = True
+
 
 def CheckChangeOnUpload(input_api, output_api):
     return input_api.canned_checks.CheckChangedLUCIConfigs(