fix(deps): update dependency django-countries to v8.1.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
django-countries (changelog)	==8.0.1 -> ==8.1.0

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Release Notes

SmileyChris/django-countries (django-countries)

v8.1.0

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Features

• DRF serializer fields now respect the allow_null parameter, returning None for NULL values when allow_null=True.
  This enables better API consistency and is particularly useful with unique=True constraints, which allow multiple NULL values but not multiple empty strings in the database. (#​453)
• Add support for null=True on multiple country fields, allowing This nullable unique constraints on multiple country fields.
  CountryField(multiple=True, null=True) now returns None for NULL database values instead of crashing and the historical system check E100 that blocked multiple=True + null=True has been removed. (#​453)

Bugfixes

• Fixed OpenAPI schema generation for CountryField when using country_dict=True or name_only=True. The field now correctly generates an object schema (with code and name properties) for country_dict=True and a string schema for name_only=True, instead of incorrectly generating an enum schema. This fixes schema generation for both DRF's built-in OpenAPI support and drf-spectacular, enabling accurate TypeScript client generation and other API tooling. (#​441)
• Major performance enhancement for Django admin. Added per-language caching to Countries.__iter__(), delivering 20-40× speedup when displaying CountryField in list_display (admin changelist now renders in <0.5s instead of 6-10s). (#​454)
• Fixed a regression where a country field allowing for selection of multiple countries could not be added to using the + operator. (#​455)

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