Support Django 4

[PeterJCLaw]

Discovered while adding CI versions for #9.
Currently the tests fail with: django.db.utils.ProgrammingError: relation "django_migrations" does not exist during export_migration_state.
I don't know if that's just a test setup issue or a genuine change in Django 4. A quick look at the release notes didn't reveal anything obvious unfortunately.

[aaron7-mns]

It's a change in Django: django/django#15252 where we were relying on django_migrations being created even when there are no migrations to run.

[PeterJCLaw]

Looks like we can just call MigrationRecorder(connection).ensure_schema() to have Django create the table? Not sure if that's meant to be public API or not, but seems like it would get things working.

[PeterJCLaw]

Hrm, adding MigrationRecorder(connection).ensure_schema() to the top of the export tests makes them pass. However I had thought that when this issue was seen under Django 4 it was seen at import time? 😕

[PeterJCLaw]

Ah, I think this only occurs at import time if there are any migration records to restore. If not then it looks like the access to django_migrations is skipped (by dint of using executemany on an empty list). That would explain why the tests aren't impacted (since they have no migration records).