Use Country (not Gender) if we're _in_south

ChoiceField sets its own `choices` property to deal with South having removed it, but this leads to an error (e.g. `'5' is not a valid choice`) if a model has a ChoiceField with more than 3 choices, and an instance with a higher-numbered choice exists in the database during a migration.

This commit uses `dj.choices.Country` instead, which means we're now good up to 215 items...