Update ART generator tools to use python3.

Test: buildbot-build.sh --target
Test: m cpp-define-generator-test && out/host/linux-x86/testcases/cpp-define-generator-test/cpp-define-generator-test
Change-Id: I73018df1e531c37d21397c2ffa35238e3df76d65
diff --git a/tools/generate_operator_out.py b/tools/generate_operator_out.py
index 91e1caf..f1491d8 100755
--- a/tools/generate_operator_out.py
+++ b/tools/generate_operator_out.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2012 The Android Open Source Project
 #