Fixed #19885 -- cleaned up the django.test namespace

* override_settings may now be imported from django.test
* removed Approximate from django.test
* updated documentation for things importable from django.test

Thanks akaariai for the suggestion.

tests/expressions_regress/tests.py

@@ -7,7 +7,8 @@ import datetime
 
 from django.db import connection
 from django.db.models import F
-from django.test import TestCase, Approximate, skipUnlessDBFeature
+from django.test import TestCase, skipUnlessDBFeature
+from django.test.utils import Approximate
 
 from .models import Number, Experiment