[3.13] gh-117657: Fix QSBR race condition (GH-118843) (#118905)

`_Py_qsbr_unregister` is called when the PyThreadState is already
detached, so the access to `tstate->qsbr` isn't safe without locking the
shared mutex. Grab the `struct _qsbr_shared` from the interpreter
instead.
(cherry picked from commit 33d20199af65c741bdc908a968edd8dc179b6974)

Co-authored-by: Alex Turner <alexturner@meta.com>