Refreeze dask-gateway/Dockerfile.requirements.txt (#789) #799

Job	Run time
Test hadoop backend	2m 13s
Test k8s backend & chart (v1.24, default)	5m 55s
Test pbs backend	1m 42s
Test k8s backend & chart (v1.25, separate-namespace)	6m 1s
Test slurm backend	1m 35s
Test k8s backend & chart (v1.26, default, 2022.4.0)	6m 6s
main-tests (3.9, 1.20)	2m 45s
main-tests (3.10, 1.21)	2m 41s
main-tests (3.11, 1.21)	2m 34s
Test k8s backend & chart (v1.27, default)	6m 3s
main-tests (3.12, 1.21)	2m 57s
Test k8s backend & chart (v1.28, default)	15s
	40m 47s

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