[1.7.x] Fixed code to solve #23070 problem

Added a class to wrap callable in settings:
* Not to call in the debug page (#21345).
* Not to break the debug page if the callable forbidding to set attributes (#23070).

Thanks @bmispelon for giving me some advice.

Backport of d0889863de from master.

tests/view_tests/tests/test_debug.py

@@ -21,7 +21,7 @@ from django.test.utils import (
     setup_test_template_loader, restore_template_loaders)
 from django.utils.encoding import force_text, force_bytes
 from django.utils import six
-from django.views.debug import ExceptionReporter
+from django.views.debug import CallableSettingWrapper, ExceptionReporter
 
 from .. import BrokenException, except_args
 from ..views import (sensitive_view, non_sensitive_view, paranoid_view,
@@ -30,6 +30,21 @@ from ..views import (sensitive_view, non_sensitive_view, paranoid_view,
     multivalue_dict_key_error)
 
 
+class CallableSettingWrapperTests(TestCase):
+    """ Unittests for CallableSettingWrapper
+    """
+    def test_repr(self):
+        class WrappedCallable(object):
+            def __repr__(self):
+                return "repr from the wrapped callable"
+
+            def __call__(self):
+                pass
+
+        actual = repr(CallableSettingWrapper(WrappedCallable()))
+        self.assertEqual(actual, "repr from the wrapped callable")
+
+
 @override_settings(DEBUG=True, TEMPLATE_DEBUG=True)
 class DebugViewTests(TestCase):
     urls = "view_tests.urls"