[3.8] bpo-43406: Fix possible race condition where ``PyErr_CheckSignals`` tries to...

[3.8] bpo-43406: Fix possible race condition where ``PyErr_CheckSignals`` tries to execute a non-Python signal handler (GH-24756) (GH-24762)

We can receive signals (at the C level, in `trip_signal()` in signalmodule.c) while `signal.signal` is being called to modify the corresponding handler.  Later when `PyErr_CheckSignals()` is called to handle the given signal, the handler may be a non-callable object and would raise a cryptic asynchronous exception..
(cherry picked from commit 68245b7a1030287294c65c298975ab9026543fd2)

Co-authored-by: Antoine Pitrou <antoine@python.org>