Unit tests fail after a Django upgrade

I am trying to bring a Django project from version 1.8 to 1.11. Pretty much everything seems to work fine except unit tests. We have a base test class inheriting from Django TestCase with a Tastypie mixin. The base class has some code in the setUp() like this

class BaseApiTest(ResourceTestCaseMixin, django.test.TestCase):
    def setUp(self):
        super().setUp()
        self.username = "secret_user"
        self.password = "sekret"
        self.email = "secret@mail.com"
        self.first_name = "FirstName"
        self.last_name = "LastName"
        self.user = User.objects.create_superuser(
            self.username,
            self.username,
            self.password
        )

And the app specific tests would inherit the base test and do something like

class TheAPITest(BaseApiTest):
    def setUp(self):
        super().setUp()
        # more setup goes here

So, under Django 1.8.x this works fine. But under 1.11.x all of these give me an error on the User.objects.create_superuser() line.

django.db.utils.InterfaceError: connection already closed

I have been going through the release notes, but there is just too much stuff that has happened between 1.8 and 1.11. Is there something simple that I am missing?