Don't report deleted attributes in __dir__ (GH#10148)

When an attribute is deleted from a Mock, a sentinel is added rather
than just deleting the attribute. This commit checks for such sentinels
when returning the child mocks in the __dir__ method as users won't
expect deleted attributes to appear when performing dir(mock).

Lib/unittest/test/testmock/testmock.py

@@ -885,6 +885,15 @@ class MockTest(unittest.TestCase):
             patcher.stop()
 
 
+    def test_dir_does_not_include_deleted_attributes(self):
+        mock = Mock()
+        mock.child.return_value = 1
+
+        self.assertIn('child', dir(mock))
+        del mock.child
+        self.assertNotIn('child', dir(mock))
+
+
     def test_configure_mock(self):
         mock = Mock(foo='bar')
         self.assertEqual(mock.foo, 'bar')