Fixed #19733 - deprecated ModelForms without 'fields' or 'exclude', and added '__all__' shortcut

This also updates all dependent functionality, including modelform_factory
 and modelformset_factory, and the generic views `ModelFormMixin`,
 `CreateView` and `UpdateView` which gain a new `fields` attribute.

tests/admin_validation/tests.py

@@ -285,6 +285,8 @@ class ValidationTestCase(TestCase):
             extra_data = forms.CharField()
             class Meta:
                 model = Song
+                fields = '__all__'
+
 
         class FieldsOnFormOnlyAdmin(admin.ModelAdmin):
             form = SongForm