[3.13] gh-119369: Fix deadlock during thread exit in free-threaded build (GH-119528) (#119868)

Release the GIL before calling `_Py_qsbr_unregister`.

The deadlock could occur when the GIL was enabled at runtime. The
`_Py_qsbr_unregister` call might block while holding the GIL because the
thread state was not active, but the GIL was still held.
(cherry picked from commit 078b8c8cf2)

Co-authored-by: Sam Gross <colesbury@gmail.com>

Python/qsbr.c

@@ -236,6 +236,11 @@ _Py_qsbr_unregister(PyThreadState *tstate)
     struct _qsbr_shared *shared = &tstate->interp->qsbr;
     struct _PyThreadStateImpl *tstate_imp = (_PyThreadStateImpl*) tstate;
 
+    // gh-119369: GIL must be released (if held) to prevent deadlocks, because
+    // we might not have an active tstate, which means taht blocking on PyMutex
+    // locks will not implicitly release the GIL.
+    assert(!tstate->_status.holds_gil);
+
     PyMutex_Lock(&shared->mutex);
     // NOTE: we must load (or reload) the thread state's qbsr inside the mutex
     // because the array may have been resized (changing tstate->qsbr) while