Throw exception on item and attribute annotations on tuple struct

[ricohageman]

Quick question on this: should we be allowing #[pyo3(item)] here? Is there any reasonable interpretation beside the elements always being item 0, item 1, item 2 etc?

I wonder if we want to consider throwing an error if #[pyo3(item)] or #[pyo3(attribute)] are used on tuple struct elements.

Originally posted by @davidhewitt in https://github.com/PyO3/pyo3/pull/2181#discussion_r814210259