Switch hftest.py to python3

Python scripts used by GN have already been moved to Python 3 in
a previous CL. hftest.py slipped through the cracks.

Change its hashbang directive and fix use of ElementTree.write()
which now expects a file name, rather than an open file object.

Change-Id: I8719acabf92d525ab0932aadf3aa0e58b4690b4c
diff --git a/test/hftest/hftest.py b/test/hftest/hftest.py
index 1c88953..17a3b25 100755
--- a/test/hftest/hftest.py
+++ b/test/hftest/hftest.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright 2018 The Hafnium Authors.
 #
@@ -521,8 +521,8 @@
             xml)
 
         # Write XML to file.
-        with open(self.artifacts.sponge_xml_path, "w") as f:
-            ET.ElementTree(xml).write(f, encoding='utf-8', xml_declaration=True)
+        ET.ElementTree(xml).write(self.artifacts.sponge_xml_path,
+            encoding='utf-8', xml_declaration=True)
 
         if result.tests_failed > 0:
             print("[x] FAIL:", result.tests_failed, "of", result.tests_run,