rework complex enum field conversion

[Icxolu]

This reworks the complex enum field conversion to use IntoPyObject either by reference if available, or together with Clone if not.

I think in the future we should probably rework complex enums to make them use of the same infrastructure we already have for #[pyo3(get)].

Closes #4216
Xref #4651

[davidhewitt]

Thanks! Agreed this could use a refactor, probably that's easier after we remove ToPyObject and other backwards-compatibility arms from the #[pyo3(get)] code.