Add `isort` to dev dependencies

[TomJGooding]

Sorry I would do this myself but I'm not yet familiar enough with poetry.

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[darrenburns]

Opened a PR for this.

For future reference, to add a development dependency in Poetry you can do:

poetry add --group dev isort

[TomJGooding]

Thanks Darren! I did check the Poetry docs but the large diff to the lock file gave me pause!

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