  from django.core.exceptions import FieldDoesNotExist
  from django.db import connections
  from django.db.models.fields import AutoField
  from django.db.models.fields.proxy import OrderWrt
  from django.utils import six
  from django.utils.datastructures import ImmutableList, OrderedSet
  from django.utils.deprecation import (
 
</s> remove from django.test import TestCase
 </s> add from django.test import TestCase, ignore_warnings </s> remove from django.db.models.manager import ensure_default_manager
 </s> add from django.db.models.manager import Manager </s> remove from django.test import TestCase
 </s> add from django.test import TestCase, ignore_warnings
from django.utils.deprecation import RemovedInDjango20Warning
from django.db.models import Manager
  from django.core.exceptions import FieldDoesNotExist
  from django.db import connections
from django.db.models import Manager
  from django.db.models.fields import AutoField
  from django.db.models.fields.proxy import OrderWrt
  from django.utils import six
  from django.utils.datastructures import ImmutableList, OrderedSet
  from django.utils.deprecation import (
keep add keep keep keep keep keep
 Fixed #10506, #13793, #14891, #25201 -- Introduced new APIs to specify models' default and base managers.

This deprecates use_for_related_fields.

Old API:

class CustomManager(models.Model):
    use_for_related_fields = True

class Model(models.Model):
    custom_manager = CustomManager()

New API:

class Model(models.Model):
    custom_manager = CustomManager()

    class Meta:
        base_manager_name = 'custom_manager'

Refs #20932, #25897.

Thanks Carl Meyer for the guidance throughout this work.
Thanks Tim Graham for writing the docs. 
django/db/models/options.py