Fix file wrapper for Django 5.2

Django 5.2 introduces (and uses) the `Model._is_pk_set()` [method](https://docs.djangoproject.com/en/5.2/ref/models/instances/#django.db.models.Model._is_pk_set), which causes an error in `FileWrapper.__getattr__()`:

```
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ERROR: test_tracked_url_generation (dev_files.tests.FileTests.test_tracked_url_generation)
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Traceback (most recent call last):
File "/home/michael/cdh/django/dsc/django-shared-core/dev/dev_files/tests.py", line 765, in test_tracked_url_generation
obj.files.add(File(open(self.file_cat, mode='rb')))
File "/home/michael/cdh/django/dsc/django-shared-core/src/cdh/files/db/wrappers.py", line 482, in add
through_obj.save()
File "/home/michael/.local/share/virtualenvs/django-shared-core-q8WVdqSE/lib/python3.12/site-packages/django/db/models/base.py", line 857, in save
self._prepare_related_fields_for_save(operation_name="save")
File "/home/michael/.local/share/virtualenvs/django-shared-core-q8WVdqSE/lib/python3.12/site-packages/django/db/models/base.py", line 1238, in _prepare_related_fields_for_save
if not obj._is_pk_set():
^^^^^^^^^^^^^^
File "/home/michael/cdh/django/dsc/django-shared-core/src/cdh/files/db/wrappers.py", line 39, in __getattr__
raise AttributeError(f"No such attribute '{item}'")
AttributeError: No such attribute '_is_pk_set'

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```

Or better said, it doesn't know what to do with it. Django 5.2 required to reproduce.