emit deprecation on pyclasses that are affected by the FromPyObject blanket impl

[Icxolu]

This emits a deprecation warning on #[pyclass]es that are affected by the FromPyObject + Clone blanket impl. It uses a Probe to detect the Clone impl and conditionally emits the deprecation. This ensures only types are flagged that are actually affected by the change. The deprecation can be suppressed by specifying either the skip_from_py_object or from_py_object option.

With this in 0.28, I think we can remove the blanket impl in 0.29.

#5419

[davidhewitt]

Thanks, generally looks great to me, nice touch to only emit it on types which are actually Clone!

I think also worth noting in the migration guide?

[davidhewitt]

I feel like it would be helpful here to make the hint a bit clearer that this is applicable to classes which implement `Clone.

Maybe something like the following (note I renamed the struct too given it shows in the public error message):

Suggested change

	warning: use of deprecated associated constant `pyo3::impl_::deprecated::DeprecatedFromPyObjectBlanket::<true>::MSG`: Implicit by value extraction of pyclasses is deprecated. Use `from_py_object` to keep the current behaviour or `skip_from_py_object` to opt-out.
	warning: use of deprecated associated constant `pyo3::impl_::deprecated::HasAutomaticFromPyObject::<true>::MSG`: The automatically derived `FromPyObject` implementation for `#[pyclass]` types which implement `Clone` is being phased out. Use `from_py_object` to keep the automatic derive or `skip_from_py_object` to accept the new behaviour.

[Icxolu]

Sounds good to me!