ipaddr library has been superseded

From https://github.com/google/ipaddr-py/blob/master/README:

```
ipaddr.py is a library for working with IP addresses, both IPv4 and IPv6.

It has been superseded by ipaddress from the Python 3 standard library, and its
Python 2 backport.
```

We should consider using ipaddress from the Python 3 standard library.