[3.13] docs: puremagic.what() as replacement for imghdr.what() (GH-120871) (#120951)

docs: puremagic.what() as replacement for imghdr.what() (GH-120871)
(cherry picked from commit 22b8a35d6e6660cf7457ed6636cb8c12fff7e8e7)

Co-authored-by: Christian Clauss <cclauss@me.com>