Update django-countries to 5.5

[pyup-bot]

This PR updates django-countries from 5.3.2 to 5.5.

Changelog

5.5

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

- Django 3.0 compatibility.

- Plugin system for extending the ``Country`` object.

5.4

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

- Renamed Macedonia -> North Macedonia.

- Fix an outlying ``makemigrations`` error.

- Pulled in new translations which were provided but missing from previous
version.

- Fixed Simplified Chinese translation (needed to be ``locale/zh_Hans``).

- Introduce an optional complex format for ``COUNTRIES_ONLY`` and
``COUNTRIES_OVERRIDE`` to allow for multiple names for a country, a custom
three character code, and a custom numeric country code.

5.3.3

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

- Add test coverage for Django Rest Framework 3.9.

Links

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