Add trait for cloning while the GIL is held

[adamreichold]

Due to issues making delay reference count increments sound, they were removed #4095 and replaced by an Clone implementation for Py which will panic without the GIL being held, gated by the py-clone feature.

This has several usability downsides, e.g. #[pyo3(get)] stops working with Py<T> fields which however is a very important use case when sharing data with Python code. Similarly, the PyBacked* types cannot be unconditionally Clone themselves.

Both problems (and likely others) should be resolvable if we add a trait, e.g. CloneRef or PyClone with signature

fn clone_ref(&self, py: Python<'_>) -> Self;

and implement it unconditionally for Py<T> and add a blanket impl based on Clone. The proc macro machinery behind #[pyo3(get)] could then go via this trait instead of the plain Clone.

[davidhewitt]

👍 I guess we probably want a derive macro for this trait too.