Support del on #[setter] defined attributes

[indygreg]

This is a follow-up to the discussion in #1775 to track implementing support for del on [setter] defined attributes.

[davidhewitt]

To clarify, implementing __delattr__ for dynamic attributes is supported and does work.

What we don't currently support is using del myclass.x when x is defined by a #[setter] or #[pyo3(set)].

TBH I'm tempted to go with "we don't need this" (after #1779 trying to delete such an attribute will raise AttributeError), however if a user does indeed have a valid use-case for this please do comment on this issue and we can discuss designs.

[davidhewitt]

As there's been no demand for a year, I'm going to close this. The comment above remains true that if a use-case emerges I'd be happy to discuss designs.