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 #2181 (comment)

[ricohageman]

@davidhewitt this seems to be resolved since #2440.
The error will read 'getter' is not permitted on tuple struct elements.

[davidhewitt]

Thanks, looks like that is indeed the case. The error message I introduced is perhaps not the greatest (could say something about item or attribute rather than getter?), however it's good enough to say this is done.