pin pydantic version in requirements_39_pandas_2_optional.txt

[LiamConnors]

Fix test failing with:

 pydantic.errors.PydanticImportError: `BaseSettings` has been moved to the `pydantic-settings` package

by pinning pydantic version. Used by vaex

See vaexio/vaex#2384

---

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

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[alexcjohnson]

💃