I came across this when trying to reproduce an unrelated bug with
someone in an outdated environment, but at least having the version
number will make us more confident when e remove the conditional branch
later.

Update some warnings with instructions and version number.

+ Fix some pep8 issues

Fix undefined variable

Found while working on napari/napari#3314

This should be the right fix, as BackgroundSocket is used only
in inprocess kernel, and while in general iopub_socket looks like it
can be `Any()` for this particular class we have a trait saying
iopub_socket has to be a BackgroundSocket

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.

In particular the in-process kernels don't.

This does not fix all the issues in particular the quit() and exit()
autocaller that become no-op, but that's another story

debugpy is an optional dependency because only frontends with
debugging support (Jupyter lab) can really use its features.

Fixes: #712

Stop using deprecated recv_multipart when using in-process socket.

Make json_clean a no-op for jupyter-client >= 7

Don't assume kernels have loops.

Make ipykernel work without debugpy