REPL and plpython

Hi!

We're using iPython as REPL for PlPython functions in Postgres. We have a PlPython function that creates and starts an iPython kernel.

With the 3.x version of iPython worked fine. I've been migrating it to the new Jupyter versions due to the big split and We've found some problems with the signal handler, more concretely [here] (https://github.com/ipython/ipykernel/blob/master/ipykernel/kernelbase.py#L245).

PLPython returns `None` when you try to get the signal handler for SIGINT but could occur in regular python too because `get_signal` could return `None` if the signal was not installed from Python. Maybe a check for `None` for the `self. saved_sigint_handler` could be a good idea?

What do you think? I was going to send a patch but first I want to be sure is treated as a possible bug