output to file fails under Python3

[Pavlo Shchelokovskyy]

I was trying to produce jUnit xml out of subunit stream.

Running "subunit2junitxml -o output.xml" with Python 3.5 (Ubuntu Xenial) fails with this traceback:

Traceback (most recent call last):
  File "/home/pshchelo/git/openstack/ironic-python-heartbeater/.tox/py35/bin/subunit2junitxml", line 36, in <module>
    protocol_version=2)
  File "/home/pshchelo/git/openstack/ironic-python-heartbeater/.tox/py35/lib/python3.5/site-packages/subunit/filters.py", line 183, in run_filter_script
    input_stream=find_stream(sys.stdin, args))
  File "/home/pshchelo/git/openstack/ironic-python-heartbeater/.tox/py35/lib/python3.5/site-packages/subunit/filters.py", line 146, in filter_by_result
    output_to = file(output_path, 'wb')
NameError: name 'file' is not defined

which is due to 'file' type was removed from Python3.

I suspect any other scripts that can write output to file are affected as well.

Obvious workaround is to use stdout to file redirection, but I still think it should be fixed since Python3 support is claimed in trove specifier.