stapi-pydantic release prep 0.0.3

[philvarner]

What I'm changing

• update version to 0.0.3 for a release in pyproject.toml and changelog
• motivation for this is being able to fix:
  • stapi-fastapi v0.7.0 depends on code in unreleased version of stapi-pydantic #104

How I did it

n/a

Checklist

• Tests pass: uv run pytest
• Checks pass: uv run pre-commit run --all-files
• CHANGELOG is updated (if necessary)