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
1 file changed