cleanup: Style improvement to python literals in make-secnet-sites

cleanup: Style improvement to python literals in make-secnet-sites

Python literals can have a trailing comma after the last element.
Doing this is a good idea as it means that adding new elements at the
end doesn't necessitate modifying the previous line.

No intentional functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>