gh-93021: Fix __text_signature__ for __get__ (GH-93023) (GH-94085)

Because of the way wrap_descr_get is written, the second argument
to __get__ methods implemented through the wrapper is always
optional.
(cherry picked from commit 4e08fbcfdfa57ea94091aabdd09413708e3fb2bf)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>