Handle Iopub welcome message for jep#65

[Hind-M]

Update jupyter_server to complete the JEP #65 implementation.

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[Hind-M]

Not sure if we should use kernel_info_timeout or something else for the timeout?

[Hind-M]

Should we add a test, maybe something similar to this? I am not sure how yet...

[Hind-M]

I think the Check Links failure in the CI is unrelated to this PR (see #1304).
Regarding the jupyterlab_server failure, is it related to some version incompatibility with jsonschema?

[Zsailer]

Hi @Hind-M 👋

Apologies for the long delay here. I'm finally getting some time to review here. Overall, I think the approach here looks good. Is there a kernel that implements the protocol version >=5.4? I couldn't seem to trigger the handshake here using the latest versions of xeus-python or ipykernel.

[Hind-M]

Hey @Zsailer!
No worries, thanks for your time!
Well, IIUC bumping to a protocol version >= 5.4 would be appreciated after getting some other features in (in addition to the XPUB one).
For now, xeus-zmq has already the XPUB corresponding PR merged (jupyter-xeus/xeus-zmq#31).
Next step would be to update the protocol version to have it in the latest version of xeus-python.
That's why I was hoping to add a test in jupyter-server with something similar to this? and check the tests locally (using the right version of xeus-zmq) while waiting for the protocol version to be out?

[Zsailer]

That's why I was hoping to add a test in jupyter-server with something similar to this?

That makes sense.

I think we should split the test_websocket_connection unit test into two different tests; and instead of calling nudge directly, we switch to using is_subscribed.

Maybe name the unit tests:

1. def test_is_subscribed_via_nudge(...)
2. def test_is_subscribed_via_xpub(...)

The rest of the unit tests are equivalent, but the configuration going into the unit tests are different to trigger the nudge/xpub flow.