Use python3

Test: warn.py build.log > warnings.html
Change-Id: I781ea7bfe023ec5af829757c417bc70aba80af0d
diff --git a/tools/warn.py b/tools/warn.py
index 22ac872..5f796f5 100755
--- a/tools/warn.py
+++ b/tools/warn.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2019 The Android Open Source Project
 #
@@ -28,7 +28,7 @@
 
 def main():
   os.environ['PYTHONPATH'] = os.path.dirname(os.path.abspath(__file__))
-  subprocess.check_call(['/usr/bin/python', '-m', 'warn.warn'] + sys.argv[1:])
+  subprocess.check_call(['/usr/bin/python3', '-m', 'warn.warn'] + sys.argv[1:])
 
 
 if __name__ == '__main__':