In the master branch of Python, trip_signal() calls _PyEval_AddPendingCall(tstate) and tstate is get using _PyRuntimeState_GetThreadState(runtime).

trip_signal() can be called while the GIL is not held: tstate is NULL in this case. For example, it's the case when calling signal.raise_signal().

Problem: _PyEval_AddPendingCall() uses tstate if pending->finishing is non-zero.

    if (pending->finishing) {
        ...
        _PyErr_Fetch(tstate, &exc, &val, &tb);
        _PyErr_SetString(tstate, PyExc_SystemError,
                        "Py_AddPendingCall: cannot add pending calls "
                        "(Python shutting down)");
        ...
    }

pending->finishing was addd in bpo-33608 by the change:

commit 842a2f07f2f08a935ef470bfdaeef40f87490cfc
Author: Eric Snow <ericsnowcurrently@gmail.com>
Date:   Fri Mar 15 15:47:51 2019 -0600

    bpo-33608: Deal with pending calls relative to runtime shutdown. (gh-12246)

I found this issue while trying to make pending calls per interpreter in bpo-39984: move pending from _PyRuntimeState to PyInterpreterState.