[3.11] gh-93575: Use correct way to calculate PyUnicode struct sizes (GH-93602) (GH-93613)

* gh-93575: Use correct way to calculate PyUnicode struct sizes

* Add comment to keep test_sys and test_unicode in sync

* Fix case code < 256.
(cherry picked from commit 5442561c1a094b68900198bade616da9ed509ac8)

Co-authored-by: Christian Heimes <christian@python.org>
3 files changed