Previously, `open()` was used with the default option of `newline=None`,
which means that “any '\n' characters written are translated to the
system default line separator”. Now, `xxd.py` always writes `\n` line
endings. This eliminates the need for the .gitattributes file.
The ability to pass a pathlib.Path to open() was new in Python 3.6,
and the oldest branch of the Steam Runtime only has Python 3.5 available.
Even without considering retrocomputing, using the Path.open method is
more consistent with how we read the input 2 lines earlier.
Signed-off-by: Simon McVittie <smcv@collabora.com>
