Upgrade three dependencies that trigger pkg_resources removal warning

[hugovk]

Description

The old versions of these three packages caused this warning when running the tests:

UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.

Removal PR: pypa/setuptools#5007.

Upgrade them.

• django-haystack 3.3.0: "Django versions 3-5 are supported, running on Python 3.8-3.12"
  https://github.com/django-haystack/django-haystack/releases/tag/v3.3.0

• django-countries 8.2.0 supports Django 3.2-5.2, Python 3.8-3.13
  https://pypi.org/project/django-countries/8.2.0/
  The 8.0.0 dropped Python 3.7
  https://github.com/SmileyChris/django-countries/blob/main/CHANGES.md#800-4-november-2025

• django-polymorphic 4.1.0: support Django 3.2-5.0, Python 3.8-3.12
  https://pypi.org/project/django-polymorphic/4.1.0/
  "There were no breaking changes in this major release"
  https://django-polymorphic.readthedocs.io/en/latest/changelog.html#v4-0-0-2025-05-20

Closes

• Nothing but helps towards ci: add native pr combiner #2592 and deps: update to django 5.2 #2741.

[JacobCoffee]

v7 to v8 was just dropped Python 3.7 support so all good to me