Refs #21831 -- Softened the TestClient dependency on contrib.auth.

This is to prevent an import of django.test causing an import (and thus
an implicit checks regisration) for an app that may not be in
`INSTALLED_APPS`.

Better fixes may be possible when #20915 and/or #21829 are addressed.

Thanks to @carljm for the report.

django/test/client.py

@@ -10,7 +10,6 @@ from io import BytesIO
 
 from django.apps import apps
 from django.conf import settings
-from django.contrib.auth import authenticate, login, logout, get_user_model
 from django.core.handlers.base import BaseHandler
 from django.core.handlers.wsgi import WSGIRequest
 from django.core.signals import (request_started, request_finished,
@@ -548,6 +547,7 @@ class Client(RequestFactory):
         are incorrect, or the user is inactive, or if the sessions framework is
         not available.
         """
+        from django.contrib.auth import authenticate, login
         user = authenticate(**credentials)
         if (user and user.is_active and
                 apps.is_installed('django.contrib.sessions')):
@@ -587,6 +587,7 @@ class Client(RequestFactory):
 
         Causes the authenticated user to be logged out.
         """
+        from django.contrib.auth import get_user_model, logout
         # Create a fake request that goes through request middleware
         request = self.request().wsgi_request