[1.6.x] Fixed #20493 -- Added a warning that objects may not be picklable across Django versions
[1.6.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

redhat-openstack/django,redhat-openstack/django,django-nonrel/django,felixjimenez/django,dex4er/django,redhat-openstack/django,dex4er/django,django-nonrel/django,redhat-openstack/django,django-nonrel/django,felixjimenez/django,felixjimenez/django,django-nonrel/django,dex4er/django,felixjimenez/django