bpo-39590: make deque.__contains__ and deque.count hold strong references (GH-18421)

Lib/test/test_deque.py

@@ -183,6 +183,18 @@ class TestBasic(unittest.TestCase):
         with self.assertRaises(RuntimeError):
             n in d
 
+    def test_contains_count_stop_crashes(self):
+        class A:
+            def __eq__(self, other):
+                d.clear()
+                return NotImplemented
+        d = deque([A(), A()])
+        with self.assertRaises(RuntimeError):
+            _ = 3 in d
+        d = deque([A(), A()])
+        with self.assertRaises(RuntimeError):
+            _ = d.count(3)
+
     def test_extend(self):
         d = deque('a')
         self.assertRaises(TypeError, d.extend, 1)