It turns out universal line ending handling is on by default in Python 3
and passing 'U' in to the mode is supposed to have no effect. See
https://docs.python.org/3/library/functions.html#open for further
information.

Unfortunately, commit fc0be4f4 which
introduced the use of argparse.FileType also put 'U' in the mode
argument. The effect it is having is causing aoix on Linux to fall
over as follows:

aoix: error: argument -o: invalid FileType('wU', encoding='UTF-8') value: 'lala.html'

This commit removes 'U' from the mode argument of FileType so that the
code will now run on Linux. The code prior to this commit ran fine on
Windows and the code as of this commit also runs fine on Windows.