Precommit: Use https://pre-commit.com/ instead?

[ghost]

Split out from #19

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Note https://pre-commit.com/ is a Python tool

And https://pre-commit.ci/ is their CI offering

We can use just the former (via GH Actions) or both.

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I've had a quick look and I think some conversion work will be needed, due to it's model of running precommit functions from other repos - (eg black, isort). That makes sense in a lot of cases (eg black, isort) but we also want to run precommit functions from this repo, and I haven't yet seen an easy way to do this. So this won't be a fast conversion, and thus I'll leave it for now.

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Found it: https://pre-commit.com/#repository-local-hooks