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.

The combination of BaseManager.from_queryset() and RenameMethodsBase results in
Manager.__module__ having the wrong value. This can be an issue when trying to
pickle the Manager class.

Signed-off-by: Jason Myers <jason@jasonamyers.com>

Refs #21102.

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

Additionally this patch solves the orthogonal problem that specialized
`QuerySet` like `ValuesQuerySet` didn't inherit from the current `QuerySet`
type. This wasn't an issue until now because we didn't officially support
custom `QuerySet` but it became necessary with the introduction of this new
feature.

Thanks aaugustin, akaariai, carljm, charettes, mjtamlyn, shaib and timgraham
for the reviews.