make-secnet-sites: Replace string.atol with int()

string.atol retured a long, I assume.  In python2, longs and ints are
distinct.  We could use long() here.  But that is not available in
python3.  Instead, write the python3 version already: just int.  We
can get an `int' that always produces longs from the python-future
module.

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