Use Python3 for presubmit (#4986)

diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index dd3117f..847deb7 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -18,6 +18,8 @@
 for more details about the presubmit API built into depot_tools.
 """
 
+USE_PYTHON3 = True
+
 LINT_FILTERS = [
   "-build/storage_class",
   "-readability/casting",