Address review: explain the purpose of the test

Co-authored-by: Victor Stinner <[email protected]>

Author: Erlend E. Aasland

Lib/test/test_array.py

@@ -42,6 +42,7 @@ def test_bad_constructor(self):
 
     @support.cpython_only
     def test_immutable(self):
+        # bpo-43908: check that array.array is immutable
         with self.assertRaises(TypeError):
             array.array.foo = 1