[3.12] gh-125984: fix use-after-free on `fut->fut_{callback,context}0` due to an evil `loop.__getattribute__` (GH-126003) (#126044)

gh-125984: fix use-after-free on `fut->fut_{callback,context}0` due to an evil `loop.__getattribute__` (GH-126003)
(cherry picked from commit f819d4301d7c75f02be1187fda017f0e7b608816)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>