move away from the FromPyObject blanket for PyClass + Clone

[Icxolu]

This proposes a first step in removing the FromPyObject blanket implementation for cloneable pyclasses.

This adds a marker trait ExtractPyClassWithClone which is automatically implemented by the #[pyclass] macro for every pyclass and modifies the blanket to require the marker. I think this should be functionally equivalent. It also adds a new skip_from_py_object option to opt out of emitting the marker trait and thus enable a custom FromPyObject implementation.

The following steps could then be:

• add an option to explicitly opt-in (this can then just emit the FromPyObject impl directly)
• emit a warning for pyclasses that do not declare out-out nor opt in
• switch the default from opt-out to opt-in
• remove the blanket and the marker and deprecate the out-out option

Ref #5419

[davidhewitt]

This approach looks good to me, thanks. Documentation with the rest of the #[pyclass] options needs updating, and somewhere in the guide probably too, maybe in the "Conversion Traits" section?

[Icxolu]

Thanks for the feedback! I've added the newsfragment, extended the pyclass option table and added a paragraph in the guide. Should we also mention this in the migration guide already, are do we wait until we emit a warning?

[davidhewitt]

Perfect, thanks!

I think no need for this to go in the migration guide until we begin emitting the warning. Users don't need to do anything with this to upgrade to 0.27.