Update django-countries to 6.0

[pyup-bot]

This PR updates django-countries from 5.5 to 6.0.

Changelog

6.0

======================

- Make DRF CountryField respect ``blank=False``. This is a backwards incompatible change since blank input will now
return a validation error (unless ``blank`` is explicitly set to ``True``).

- Fix ``COUNTRIES_OVERRIDE`` when using the complex dictionary format and a single name.

- Add bandit to the test suite for basic security analysis.

- Drop Python 2.7 and Python 3.4 support.

- Add Rest Framework 3.10 and 3.11 to the test matrix, remove 3.8.

- Fix a memory leak when using PyUCA. Thanks Meiyer (aka interDist)!

Links

• PyPI: https://pypi.org/project/django-countries
• Changelog: https://pyup.io/changelogs/django-countries/
• Repo: https://github.com/SmileyChris/django-countries/

[pyup-bot]

Closing this in favor of #1784