setUpClass is not called with either pytest>4.1.1 or pytest-django>3.4.6

This is similar to #710 and #624

These 2 combinations work for me on python2:

pytest==4.1.1
pytest-django==3.5.1

pytest==4.6.4
pytest-django==3.4.6

But I cannot run the 2 libraries together on their latest release as in one test method out of a test suite with more than 5000 unit tests it occurs that the setUpClass method for the one test is then not called. The setUpClass is defined in a widely used mixin class (not inheriting from a TestClass base) and works everywhere else.

It is really weird how two testcases seem to disturb one another but it does not make any sense to me:

Here a quick description of my scenario:

mixin.py
class MyMixin() with a setUpClass

firstmodule.py:
class FirstTestCase(MyMixin) with test_foo

secondmodule.py

from firstmodule import FirstTestCase <---- when commenting this out my setUpClass gets run again for FirstTestCase::test_foo
class OtherModulesTestCase(FirstTestCase)

Start by reproducing the issue with the stated pytest and pytest-django version combinations, using mixin.py, firstmodule.py, and secondmodule.py as the minimal scenario. Trace how importing FirstTestCase and subclassing it affects setUpClass; done means the method is reliably called for FirstTestCase::test_foo in the affected combinations.