fix client

[edublancas]

to enable stdin, we had to modify nbclient; however, since nbclient does not have to support the stdin channel, the code doesn't set a timeout for the channels, since it assumes all messages will return a response via the shell and io_pub; this causes out modification to hang up. The quick fix is to add a little time.sleep call to ensure that cells asking for input are correctly handled; but we should fix this

we can use this as inspiration: https://github.com/jupyter/jupyter_console/blob/main/jupyter_console/ptshell.py