Better order tests when mixing transactional tests and Django test classes

[pmourlanne]

Closes #827

• Edited test_db_order to add Django test classes (test was failing, as expected)
• Changed pytest_collection_modifyitems to take Django classes into consideration (test is now passing)
• Fix test_django_not_loaded_without_settings

About test_django_not_loaded_without_settings

test_django_not_loaded_without_settings is now failing. That's not a surprise since we import django in plugin.py. Moving the import in the pytest_collection_modifyitems does not work either (since the function is run when collecting tests 🙃 )
I tried looking for eg. a class attribute unique to TestCase or TransactionTestCase but I didn't find any (not that it would be a great solution :)

Another solution could be to dynamically import django in pytest_collection_modifyitems after checking for DJANGO_SETTINGS_MODULE? Please advise if you have other / better ideas :)

I ended up doing the dynamic import, taking inspiration from other places where django_settings_is_configured is used. It's a bit ugly but it does the job :O Improvements welcome :)

[blueyed]

Nice, thanks!
Left some comments/ideas.

[pmourlanne]

I should have taken your comments into account, tell me if you still see things to improve / change 🕵️‍♀️

[blueyed]

Awesome, thanks!