Require `jupyter-server-proxy` 4.4.0 or newer #290

Job	Run time
build-test-images (jupyterlab, gpu)	3m 44s
build-test-images (jupyterlab, cpu)	3m 24s
build-test-images (jupyterhub, cpu)	1m 44s
build-test-images (dask-worker, gpu)	1m 32s
build-test-images (dask-worker, cpu)	1m 36s
	12m 0s

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