Close output file to ensure all buffered output actually gets written.
Otherwise, glib-genmarshal output is sometimes empty (for example, when trying
to build gdk-pixbuf on Windows, with Meson installed from .msi package).
argparse.FileType doesn't get closed automagically when the script exits:
https://bugs.python.org/issue13824
Fixes https://gitlab.gnome.org/GNOME/glib/-/issues/2341