gh-91803: Mock - fix error when using autospec methods with seal (#92213)

Fixes https://github.com/python/cpython/issues/91803.

Co-authored-by: Karthikeyan Singaravelan <tir.karthi@gmail.com>
Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>

Lib/unittest/mock.py

@@ -2745,6 +2745,7 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
                               _new_parent=parent,
                               **kwargs)
             mock._mock_children[entry] = new
+            new.return_value = child_klass()
             _check_signature(original, new, skipfirst=skipfirst)
 
         # so functions created with _set_signature become instance attributes,