Refreeze dask-gateway-server/Dockerfile.requirements.txt #848

Job	Run time
Test hadoop backend	2m 15s
Test k8s backend & chart (v1.25, separate-namespace)	5m 58s
Test pbs backend	1m 48s
main-tests (3.9, 1.20)	2m 25s
Test k8s backend & chart (v1.26, default, 2022.4.0)	6m 15s
Test slurm backend	1m 34s
main-tests (3.10, 1.21)	2m 27s
Test k8s backend & chart (v1.27, default)	6m 1s
main-tests (3.11, 1.21)	2m 21s
Test k8s backend & chart (v1.28, default)	5m 49s
main-tests (3.12, 1.21)	2m 32s
Test k8s backend & chart (v1.29, default)	5m 47s
	45m 12s

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