Pickling a `SimpleLazyObject` wrapping a model did not work correctly; in
particular it did not add the `_django_version` attribute added in 42736ac8.
Now it will handle this and other custom `__reduce__` methods correctly.

Thanks KS Chan for the report and Tim Graham for the review.

Fixed in 97774429.

Thanks Daniel Hahler for the initial patch.

Replaced occurrences of os.path.join(BASE_DIR, 'folder/subfolder')
to os.path.join(BASE_DIR, 'folder', 'subfolder')

Fixed in 97774429.

Appears unused since 585b7aca.

This is consistent with ability to reference other descriptors
on the model class (5ef0c03a).

Thanks Simon Charette for the DeprecationInstanceCheck class.

Thanks Anjul Tyagi for some of the draft text.

This avoids creating an empty list which is invalid HTML 4.

Thanks Stavros Korokithakis for the report and Tim Graham for the
review.

Thanks Tim Graham for the patch series review.