ci: Add testing against django main

[marksweb]

This adds testing against django's main branch.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ecac82f) 90.88% compared to head (9bcb377) 90.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #353 +/- ## ======================================= Coverage 90.88% 90.88% ======================================= Files 72 72 Lines 2546 2546 Branches 361 361 ======================================= Hits 2314 2314 Misses 168 168 Partials 64 64 ```

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[fsbraun]

@marksweb You'll need to also swap installation of requirements and django main branch.

[fsbraun]

@marksweb Should we test the Django main branch with django CMS develop-4 ?

[fsbraun]

@marksweb Does it make sense to run the py3.12 against develop-4 for the time being?

[marksweb]

@fsbraun not if support might take a while.

[fsbraun]

I think it's there.

[fsbraun]

@marksweb I added the following lines to the three test files that use assertQuerySetEqual just after the imports:

if not hasattr(CMSTestCase, "assertQuerySetEqual"):
    # Django < 4.2
    CMSTestCase.assertQuerySetEqual = CMSTestCase.assertQuerysetEqual

Might be easier to remove later...