Prefer field_path for admin filter to follow relations

[SebCorbin]

When filtering on a CountryField from a relation, the admin will break with a FieldError: Cannot resolve keyword 'country' into field. Choices are: ...

Full example code:

# models.py
class Contact(models.Model):
    country = CountryField(
        verbose_name=_("Country"),
        blank=True,
        null=True,
    )


class Payment(models.Model):
    contact = models.ForeignKey(
        "myapp.Contact", verbose_name=_("Contact"), on_delete=models.RESTRICT
    )

# admin.py
class PaymentAdmin(ModelAdmin):
    model = Payment
    list_filter = [
        ("contact__country", CountryFilter),
    ]