Upgrade django-synchro for django 2.2 : OperationalError - no such table: django_content_type" - django-2.2

Question

I am trying to upgrade django-synchro to django 2.2. I have already upgraded the project to django 2.1 but I have now a problem with ContentType object
The upgraded version to django 2.1 can be found here
(python runtests.py works, all tests pass)
With django 2.2.3 I have the error
django.db.utils.OperationalError: no such table: django_content_type
It seems that at initialisation, migrations are done on default database and the rest it is done on test database (in memory). So ContentType are not seen in the rest of the code. An error occur when models.py is read
(content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE))
Any ideas would be very appreciated...
I have looked in Django 2.2 release notes
There are two backwards incompatible changes in 2.2 that can maybe do the error :
TransactionTestCase serialized data loading : Initial data
migrations are now loaded in TransactionTestCase at the end of the
test, after the database flush. In older versions, this data was
loaded at the beginning of the test, but this prevents the test
--keepdb option from working properly (the database was empty at the end of the whole test suite). This change shouldn’t have an impact on
your tests unless you’ve customized TransactionTestCase’s internals.
Test : Deferrable database constraints are now checked at the end of
each TestCase test on SQLite 3.20+, just like on other backends that
support deferrable constraints. These checks aren’t implemented for
older versions of SQLite because they would require expensive table
introspection there.

Answer 1

For me, it was this note from the Django 2.2 release notes
Tests will fail on SQLite if apps without migrations have relations to
apps with migrations. This has been a documented restriction since
migrations were added in Django 1.7, but it fails more reliably now.
You’ll see tests failing with errors like no such table:
_. This was observed with several third-party apps
that had models in tests without migrations. You must add migrations
for such models.
I'm not sure if the error message is particularly helpful. But a round of upgrades and checking that I had no really old django modules lying around seemed to fix it.