Fully drop support for python 2 in chromeos test_runner.

This'll make the main entry-point (test_runner.py) error out if it
detects it's running on py2.

Bug: 1198282
Change-Id: I797dcb9e36184b9f7e66ba5555394789a5c59c13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2847242
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#875896}

build/chromeos/PRESUBMIT.py

@@ -13,7 +13,11 @@ def CommonChecks(input_api, output_api):
   results += input_api.canned_checks.RunPylint(
       input_api, output_api, pylintrc='pylintrc')
   tests = input_api.canned_checks.GetUnitTestsInDirectory(
-      input_api, output_api, '.', [r'^.+_test\.py$'], run_on_python3=True)
+      input_api,
+      output_api,
+      '.', [r'^.+_test\.py$'],
+      run_on_python2=False,
+      run_on_python3=True)
   results += input_api.RunTests(tests)
   return results
 

build/chromeos/test_runner.py

@@ -24,6 +24,8 @@
 import psutil  # pylint: disable=import-error
 import six
 
+assert not six.PY2, 'Use of this test runner with python 2 is not supported.'
+
 CHROMIUM_SRC_PATH = os.path.abspath(
     os.path.join(os.path.dirname(__file__), '..', '..'))