Use python3

Test: warn.py build.log > warnings.html
Change-Id: I781ea7bfe023ec5af829757c417bc70aba80af0d
diff --git a/tools/warn/warn.py b/tools/warn/warn.py
index 56e8787..cb5daec 100755
--- a/tools/warn/warn.py
+++ b/tools/warn/warn.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2019 The Android Open Source Project
 #