[3.13] gh-126080: fix UAF on `task->task_context` in `task_call_step_soon` due to an evil `loop.__getattribute__` (GH-126120) (#126250)

gh-126080: fix UAF on `task->task_context` in `task_call_step_soon` due to an evil `loop.__getattribute__` (GH-126120)
(cherry picked from commit 0e8665554b2f1334e530fd6de5b3a4e908405419)

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