Stop using deprecated recv_multipart when using in-process socket.

[Carreau]

Found while working on napari/napari#3314

I'm not too sure this is the right fix, as BackgroundSocket is used only
in inprocess kernel, and that in general io_pub socket looks like the
can be Any() and that therefore may not have a io_thread.

But if that's the case, then should we maybe un-deprecate recv_multipart on background socket ?

the recv in jupyter_client side (which is called by the line I change
here) is

def recv(self, socket, mode=zmq.NOBLOCK, content=True, copy=True):
    """Receive and unpack a message.

    Parameters
    ----------
    socket : ZMQStream or Socket
        The socket or stream to use in receiving.

    Returns
    -------
    [idents], msg
        [idents] is a list of idents and msg is a nested message dict of
        same format as self.msg returns.
    """
    if isinstance(socket, ZMQStream):
        socket = socket.socket
    try:
        msg_list = socket.recv_multipart(mode, copy=copy) # this will trigger deprecation warning
    except zmq.ZMQError as e:
        ...

And I doubt we want to make that aware of background socket.

[Carreau]

I believe @minrk and or @SylvainCorlay might have more experience around this area.

[minrk]

Fine with undeprecating recv_multipart, or adding a check here that does an isinstance if it's a socket itself or a BackgroundSocket.

[Carreau]

or adding a check here that does an isinstance if it's a socket itself or a BackgroundSocket.

Actually while doing that I just realized that

class InProcessKernel(IPythonKernel):

    ...

    iopub_socket = Instance(BackgroundSocket)

So it can only be a BackgroundSocket or subclass so the isinstance seem unnecessary and this is likely the right fix.

[Carreau]

FOrce pushed identical commit but with above note added to the text.