Historically, Djblets has used the publicsuffix module to compute the

correct public sufix for a domain (e.g., example.com from

www.example.com). This module is now deprecated, replaced by a modern

publicsuffixlist module, which has wider Python support and a built-in

data file.
We now use this module, which is fairly close to a drop-in replacement

(it actually has a compatible interface for publicsuffix users, but

it's trivial to migrate to the modern API).
We also now cache the instance, to avoid having to load and parse every

time we're checking a DMARC record.

Unit tests pass.