Fixed #21430 -- Added a RuntimeWarning when unpickling Models and QuerySets from a different Django version.

Thanks FunkyBob for the suggestion, prasoon2211 for the initial patch,
and akaariai, loic, and charettes for helping in shaping the patch.

Thanks Piotr Kasprzyk for help with the patch.

Refs #17262.

Should be unneeded with Python 2.7 and up.
Added some unicode_literals along the way.

Refs #20680.

This test hits a bug in current ports of MySQLdb.