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* Add the attributes to ``Countries`` class that can override the default
settings.
* CountriesField can now be passed a custom countries subclass to use, which
combined with the previous change allows for different country choices for
different fields.
* Allow ``COUNTRIES_ONLY`` to also accept just country codes in its list
(rather than only two-tuples), looking up the translatable country name from
the full country list.
* Fix Montenegro flag size (was 12px high rather than the standard 11px).
* Fix outdated ISO country name formatting for Bolivia, Gambia, Holy See,
Iran, Micronesia, and Venezuela.