Bump pydantic to get bugfix! (#75)

Why
===

See #65 for context, or
pydantic/pydantic-core#1332 directly.

The issue we were seeing was specifically around union decoders failing
to decode properties of the selected variant, so even though we'd get
the right model it wouldn't be filled out.

What changed
============

Bump pydantic to the latest released beta

Test plan
=========

Run codegen, observe that the generated code matches the previously
generated code to a reasonable degree.