Django-countries-with-calling-codes

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- Fix some translations that were included in 5.1 but not compiled.

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* Tests now also cover Django Rest Framework 3.7 and Django 2.0.

* Allow for creating country fields using (valid) alpha-3 or numeric codes.

* Fix migration error with blank default (thanks Jens Diemer).

* Add a ``{% get_countries %}`` template tag (thanks Matija Čvrk).

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* No longer allow ``multiple=True`` and ``null=True`` together. This causes
problems saving the field, and ``null`` shouldn't really be used anyway
because the country field is a subclass of ``CharField``.

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* Use transparency layer for flag sprites.

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* Fix invalid reStructuredText in CHANGES.

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* Add a ``CountryFieldMixin`` Django Rest Framework serializer mixin that
automatically picks the right field type for a ``CountryField`` (both single
and multi-choice).

* Validation for Django Rest Framework field (thanks Simon Meers).

* Allow case-insensitive ``.by_name()`` matching (thanks again, Simon).

* Ensure a multiple-choice ``CountryField.max_length`` is enough to hold all
countries.

* Fix inefficient pickling of countries (thanks Craig de Stigter for the report
and tests).

* Stop adding a blank choice when dealing with a multi-choice ``CountryField``.

* Tests now cover multiple Django Rest Framework versions (back to 3.3).