[1.5.x] Fixed #20493 -- Added a warning that objects may not be picklable across Django versions
[1.5.x] Fixed #20493 -- Added a warning that objects may not be picklable across Django versions

Thanks cataliniacob for the suggestion and review.

Backport of 1084575163 from master

ccn-2m/django,bliti/django-nonrel-1.5,bliti/django-nonrel-1.5,imtapps/django-imt-fork,alx-eu/django,alx-eu/django,bliti/django-nonrel-1.5,imtapps/django-imt-fork,hasadna/django,imtapps/django-imt-fork,ccn-2m/django,ccn-2m/django,hasadna/django,ccn-2m/django,alx-eu/django,hasadna/django,alx-eu/django