ci: fix ci failure, optimize workflow triggers, document use of pre-commit.ci

[consideRatio]

Ignore this PR until having considered #476!

If there is agreement on using the pre-commit.ci service as motivated within #476, then this PR closes #476.

[consideRatio]

pre-commit.ci / dependabot pushes to these branches and opens a PR. So like this one would avoid triggering two workflow executions - one for the pushed branch, one for the PR.

[martindurant]

I do like normalising how things are done, and of course fast failures - all without worrying about upstream checker versions. Does this add any extra effort when developing locally?

[consideRatio]

Does this add any extra effort when developing locally?

Using pre-commit.ci would remove one piece of extra effort of developing locally and submitting a PR, which is to install and run pre-commit, which in turn run the black autoformatting hook as configured already in this repo. This is because pre-commit.ci would push a commit with the autoformatting changes needed to the PR if you had not done it yourself locally.

Whats critical and should be fixed no matter what, is that we now have a .pre-commit-config.yaml referencing an outdated version of black, and then we run tests with a new version of black in the github workflow.

[martindurant]

I see what you mean :)

would reformat dask-gateway/dask_gateway/options.py
would reformat dask-gateway-server/dask_gateway_server/backends/jobqueue/pbs.py
would reformat dask-gateway-server/dask_gateway_server/backends/jobqueue/slurm.py
would reformat dask-gateway-server/dask_gateway_server/backends/kubernetes/utils.py
would reformat dask-gateway-server/dask_gateway_server/traitlets.py
would reformat dask-gateway-server/dask_gateway_server/workqueue.py
would reformat dask-gateway-server/dask_gateway_server/utils.py
would reformat tests/kubernetes/test_methods.py
would reformat tests/test_pbs_backend.py
would reformat tests/test_slurm_backend.py
would reformat tests/test_utils.py

Oh no! 💥 💔 💥
[11](https://github.com/dask/dask-gateway/runs/5168260835?check_suite_focus=true#step:9:11) files would be reformatted, 57 files would be left unchanged.

[jacobtomlinson]

Test failures here are unrelated. Merging.