Stopped populating the app registry as a side effect.

Since it triggers imports, it shouldn't be done lightly.

This commit adds a public API for doing it explicitly, django.setup(),
and does it automatically when using manage.py and wsgi.py.

django/core/serializers/base.py

@@ -3,7 +3,6 @@ Module for abstract serializer/unserializer base classes.
 """
 import warnings
 
-from django.apps import apps
 from django.db import models
 from django.utils import six
 
@@ -137,9 +136,6 @@ class Deserializer(six.Iterator):
             self.stream = six.StringIO(stream_or_string)
         else:
             self.stream = stream_or_string
-        # Make sure the app registy is loaded before deserialization starts
-        # (otherwise subclass calls to get_model() and friends might fail...)
-        apps.populate_models()
 
     def __iter__(self):
         return self

django/core/serializers/python.py

@@ -88,8 +88,6 @@ def Deserializer(object_list, **options):
     db = options.pop('using', DEFAULT_DB_ALIAS)
     ignore = options.pop('ignorenonexistent', False)
 
-    apps.populate_models()
-
     for d in object_list:
         # Look up the model and starting build a dict of data for it.
         Model = _get_model(d["model"])