relation_set broken under Python3

Due to Juju not reporting version information, relation_set needs to sniff to see if the --file argument is available to relation-set or not. Unfortunately, the code that does this is broken under Python 3.

accepts_file = "--file" in subprocess.check_output(
        relation_cmd_line + ["--help"])

subprocess.check_output returns a byte string by default, and the 'in' operator fails. To fix this, add universal_newlines=True to the subprocess.check_output call, or decode the output using the system encoding.