gh-58211: Add tests for __self__ attribute of builtins functions

Doc/library/array.rst

@@ -266,4 +266,3 @@ Examples::
 
    `NumPy <https://numpy.org/>`_
       The NumPy package defines another array type.
-

Lib/test/test_funcattrs.py

@@ -473,6 +473,26 @@ def test_builtin__qualname__(self):
         self.assertEqual([1, 2, 3].append.__qualname__, 'list.append')
         self.assertEqual({'foo': 'bar'}.pop.__qualname__, 'dict.pop')
 
+    def test_builtin__self__(self):
+        import time
+        import builtins
+
+        # builtin function:
+        self.assertEqual(len.__self__, builtins)
+        self.assertEqual(time.sleep.__self__, time)
+
+        # builtin classmethod:
+        self.assertEqual(dict.fromkeys.__self__, dict)
+        self.assertEqual(float.__getformat__.__self__, float)
+
+        # builtin staticmethod:
+        self.assertEqual(str.maketrans.__self__, None)
+        self.assertEqual(bytes.maketrans.__self__, None)
+
+        # builtin bound instance method:
+        self.assertEqual([1, 2, 3].append.__self__, [1, 2, 3])
+        self.assertEqual({'foo': 'bar'}.pop.__self__, {'foo': 'bar'})
+
 
 if __name__ == "__main__":
     unittest.main()