deps: update to pydantic 2.0 using built in back-compat

[sambarnes]

closes #39

pydantic includes a ref to v1 for back-compat, so this felt straight forward enough

https://docs.pydantic.dev/2.0/migration/#continue-using-pydantic-v1-features

tests look green locally, but lmk if ya have any fundamental concerns with the approach! 🙇

[sbihel]

Thank you