jupyter_client 5.3.0 release

[Matthew Seal]

Hello Jupyter devs and users,

We had some long running threads for a few features that needed a jupyter_client version bump to support, so we put together this small release to accommodate the need. The largest changes in 5.3.0 are the removal of older python 3 version support and the thread / process safe improvements to jupyter client.

For more details see: https://discourse.jupyter.org/t/jupyter-client-5-3-0-release/1584

Best,

Jupyter Team