build: Add wheel `task`, use in CI

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This lets us build & test locally

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Thanks for reviewing — I should probably either finish this or merge it, sorry for leaving it hanging for so long...

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Note to future self (or if someone else finishes this) — we should add the ability to use maturin develop, since that installs the python package in a single step.

CI etc uses maturin build + pip install, which is a more complete installation path and better simulates how it'll be used, but requires an extra step + a path change

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Most of this was added in a separate PR