CountryField shows up as "-" in admin when readonly

[dtcooper]

Hi there,

When a CountryField with multiple=True on a model has an associated ModelAdmin that is readonly for whatever reason (insufficient permissions or the field is marked in readonly_fields), it shows up in the admin UI as "-" no matter what.

Steps to reproduce:

1. Create a model with a CountryField with multiple=True and associated readonly ModelAdmin,

# myapp/models.py
from django.db import models
from django_countries.fields import CountryField

class Test(models.Model):
    countries = CountryField(multiple=True)

# myapp/admin.py
from django.contrib import admin
from myapp.models import Test

class TestAdmin(admin.ModelAdmin):
    readonly_fields = ("countries",)

admin.site.register(Test, TestAdmin)

2. Add multiple countries to the model

# In ./manage.py shell
from myapp.models import Test

test = Test(countries=["US", "CA"])
test.save()

3. In Django's admin site observe something like this,

Note, what's expected is to see the list of countries render as readonly, perhaps as a comma-separated string?

[dtcooper]

btw thanks for the great package! :)