bin/gen_calendar_entries: fix newlines on windows

bin/gen_calendar_entries: fix newlines on windows

The documentation[1] for the csv module specifies that we should specify
newline='' when opening the output file. Without that, the module
garbles the newlines, writing them as \r\r\n on Windows instead of \r\n.

So let's do what the documentation says, and specify newline=''

[1]: https://docs.python.org/3/library/csv.html#id3

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12405>