[3.10] gh-100931: Test all pickle protocols in test_slice (GH-100932

). (GH-100978)

(cherry picked from commit 8795ad1)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

Automerge-Triggered-By: GH:AlexWaygood

Lib/test/test_slice.py

@@ -236,8 +236,10 @@ def __setitem__(self, i, k):
         self.assertEqual(tmp, [(slice(1, 2), 42)])
 
     def test_pickle(self):
+        import pickle
+
         s = slice(10, 20, 3)
-        for protocol in (0,1,2):
+        for protocol in range(pickle.HIGHEST_PROTOCOL + 1):
             t = loads(dumps(s, protocol))
             self.assertEqual(s, t)
             self.assertEqual(s.indices(15), t.indices(15))