Add support for Django 5.2

[browniebroke]

Django 5.2 is due to be released in a few weeks: https://thib.me/django-packages-already-on-5-2

• [x] Update tox config to run tests against 5.2
• [x] Update Trove classifiers to declare support for 5.2

[codecov[bot]]

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

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[FlipperPA]

@browniebroke Thanks so much for being on top of this! Feel free to merge whenever you feel its ready - I'm guessing that'll be on 5.2 release.