make-secnet-sites etc.: Use unicode

We are going to want to switch to ipaddress from ipaddr, since
ipaddress is available in python3.  But ipaddress insists on unicode
strings, even in python2.  ipaddr doesn't mind them.

So make everything be unicode.  In particular: all of our literals and
all of our io streams.  We wrap up io.open(), which is a compatibility
thing from python-future.

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