perfetto: migrate PRESUBMIT.py to Python 3

Fix some issues found by git cl presubmit --all

Bug: 258514344
Change-Id: Ia57b869e751b4673a5bdf807d9c634f606453e52
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 5307e5a..09fa05c 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -18,6 +18,9 @@
 import time
 
 
+USE_PYTHON3 = True
+
+
 def RunAndReportIfLong(func, *args, **kargs):
   start = time.time()
   results = func(*args, **kargs)