[3.13] gh-75572: Forward-port test_xpickle from Python 2 to 3 (GH-22452) (GH-143509)

Move data classes used in tests to separate file test_picklecommon.py,
so it can be imported in old Python versions.
(cherry picked from commit 8735daf3e82d50defd791e9be7b9ae6843bc4ed1)
(cherry picked from commit ff0a8b72892e94ef6616ed70241d23985f7a5ac4)

Co-authored-by: Ken Jin <kenjin@python.org>