Fixed #21430 -- Warning raised when unpickling Models and QuerySet from different versions

Thanks @charettes @akaariai @loic for help.
Pickling models and querysets across different versions of Django isn't supported. An extra variable that specifies the current Django version is now added to the pickled state of models and querysets, and Django raises a RuntimeWarning when these objects are unpickled from a different version than the one in which they were pickled.