Update python to 3.12

[gilligan]

Description

Provide python 3.12 in the nix shell (previously we were on 3.11)

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• New tests are added if needed and existing tests are updated.
• Relevant logging and metrics added
• CI passes. See note on CI.
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

Note on CI

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You will need to get someone with write privileges. Please contact IOG Partner Chains developers to do this
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[gilligan]

At this point I'm not entirely sure if I shouldn't just remove python from the nix shell completely seeing how it's currently causing problems with the use of numpy anyway.
I'm leaning towards removing it and just assuming that python needs to be installed individually.
@rsporny you probably don't mind either way?

[rsporny]

@gilligan I don't mind, I checked out this branch and it works fine. I'll switch to using devshell's python if we merge it.