debug-build-stacktrace.txt:
* Thread 1 is waiting on threading_hang.py:21: thread.join()
* Thread 2-6 (5 threads) are waiting for the GIL

The thread 1 is not supposed to hold the GIL: thread.join() is implemented by lock_PyThread_acquire_lock() at Modules/threadmodule.c:52:

    Py_BEGIN_ALLOW_THREADS
    i = PyThread_acquire_lock(self->lock_lock, i);
    Py_END_ALLOW_THREADS

Note: I backported enhancements of python-gdb.py from Python 3, but I'm not sure that "Waiting for the GIL" line is reliable :-/ I recall vaguely a technical issue specific to Python 2 to check if a thread is waiting on the GIL or not.