enable older ipaddr module, for compatibility with older versions of python

Python only added the ipaddress module to its standard library in
3.3, but some older systems will still be using older versions of
python3 for some time. (e.g. debian wheezy, which will probably be
supported into 2016, has python 3.1).

For these older systems, the simplest route is to install the ipaddr
module, which has slightly different naming schemes for some
functions.

For systems already running python 3.3, this patch won't change their
behavior, but it will make it possible to run py-postgresql against
python 3.1, as long as the external ipaddr module is also installed
separately.