bpo-43908: Make array.array type immutable (GH-25696)

Co-authored-by: Victor Stinner <vstinner@python.org>
diff --git a/Lib/test/test_array.py b/Lib/test/test_array.py
index bdcd125..11184ad 100644
--- a/Lib/test/test_array.py
+++ b/Lib/test/test_array.py
@@ -40,6 +40,12 @@ def test_bad_constructor(self):
         self.assertRaises(TypeError, array.array, 'xx')
         self.assertRaises(ValueError, array.array, 'x')
 
+    @support.cpython_only
+    def test_immutable(self):
+        # bpo-43908: check that array.array is immutable
+        with self.assertRaises(TypeError):
+            array.array.foo = 1
+
     def test_empty(self):
         # Exercise code for handling zero-length arrays
         a = array.array('B')