Use pydantic instead of JSON Schema validation where appropriate

[jpmckinney]

https://docs.pydantic.dev/latest/

We can use it in:

• [x] Kingfisher Collect (to check pipeline items) (kingfisher_scrapy/item_schema)
• [ ] extension registry tests (to check CSV rows) (schema)
• [ ] standard-maintenance-scripts tests (to check CSV rows) (schema)

The following use OCDS' schema, and so their use of jsonschema is appropriate:

• jscc (to check JSON files in documentation)
• lib-cove-ocds (to check OCDS data)
• ocds-merge (to check OCDS fixtures)
• sample-data (to check OCDS data)