Consider adding default_auto_field to AppConfig

[amartinezuy]

Hello,

Starting with Django 3.2, newly created projects will include the following line in settings.py:

DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'

When installing django-invitations in such a context and running python manage.py migrate we get the following message:

Your models in app(s): 'invitations' have changes that are not yet reflected in a migration, and so won't be applied.
Run 'manage.py makemigrations' to make new migrations, and then re-run 'manage.py migrate' to apply them.

And running makemigrations would create a migration like this:

operations = [
    migrations.AlterField(
        model_name='invitation',
        name='id',
        field=models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID'),
    ),
]

But, naturally, this migration would be created in the packages installation directory (usually a virtualenv). Which is undesirable behavior.

There are ways to patch this from outside the lib, but maybe it would be a good idea to modify invitations/apps.py to look like this:

from django.apps import AppConfig


class Config(AppConfig):
    """Config."""

    default_auto_field = 'django.db.models.BigAutoField'
    name = "invitations"
    label = "invitations"