Fixed #21853 -- Fixed Manager.__module__ to properly return 'django.db.models.manager'.

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.
Fixed #21849 -- Included the count of silenced system checks in output.
Add an extra warning about custom save methods in migrations docs.
Fixed #21883 -- Added six.moves.xrange import

When moving code around from sql.where to lookups six.moves.xrange
import was forgotten. The xrange import is needed on Python 3.
Added tests for m2m queries with custom pk on the end models

It seems this case was fixed somewhere between 1.5.x and 1.6.x. I added
tests as I wasn't able to find any tests for these cases. Refs #21879