[tharropoulos]

• add SynonymSetItems and SynonymSetItem (sync and async) to support synonym_sets[name].items[id].retrieve(), .upsert() and .delete(), matching the docs snippets that currently raise AttributeError
• add CurationSetItems and CurationSetItem (sync and async) with the same shape for curation_sets[name].items[id]
• expose items.retrieve(limit=, offset=) for listing and items.upsert(id, item) as a shortcut for upserting by id
• split SynonymItemUpsertSchema and CurationItemCreateSchema out as the body types without id, since the item endpoint takes the id from the path and mypy rejected the docs snippet
• keep SynonymItemSchema and CurationItemUpsertSchema unchanged by making them subclasses that add id
• route list_items, get_item, upsert_item and delete_item through the new accessor and widen upsert_item to accept bodies with or without id
• add sync and async tests for the accessor paths and for the existing flat methods in tests/synonym_set_items_test.py and tests/curation_set_test.py

[haydenhoang]

lgtm!