make-secnet-sites: Switch to `ipaddress' from `ipaddr'

make-secnet-sites: Switch to `ipaddress' from `ipaddr'

ipaddress is available in python3 and ipaddr is not.

Code changes:
 - Change the imports and references to the module name
 - IPNetwork & IPAddress functions => ip_address & ip_network
 - There is no IPNetwork superclass so don't mention it in docstrings
 - collapse_address_list => collapse_addresses
 - There is no version parameter to ip_address; we have to
   switch on v ourselves and call IPv6Address or IPv4Address

Administrivia:
 - Update debian/control and INSTALL.
 - Remove references to ipaddr's licence.  ipaddress is under
   the same licence as python so does not need special mention.

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