[3.11] [3.12] gh-108314: PyDict_GetItemString() mentions UTF-8 (GH-108448) (#108489)

[3.12] gh-108314: PyDict_GetItemString() mentions UTF-8 (GH-108448)

gh-108314: PyDict_GetItemString() mentions UTF-8

PyDict_GetItemString(), PyDict_SetItemString() and
PyDict_DelItemString() expects a UTF-8 encoding string for the key.
(cherry picked from commit 9a225d7d5b0530ee73fa00d4816897997a9eb733)

Co-authored-by: Victor Stinner <vstinner@python.org>