Pickle support for PyErr

[TennyZhuang]

Pickle support was already discussed in #100, customizing __setstate__, __getstate__ and __getnewargs_ex__ works well for customized pyclass.

Unfortunately, when we want to create customized exception using create_exception, it's hard to implement such methods. It seems that the arguments are stored in PyErr instead of the class itself, how can we implement these magic methods ourselves?

#[pyclass(extends=PyException, subclass)]
pub struct Error;

Of course, it's better for pyo3 to support these methods directly on PyErr. I'd like to help if there is a consensus to do that.

[Icxolu]

I would say the goal is to make exceptions work exactly like your snippet. Create a custom pyclass and inherit from an exception type. That way it's easy to add custom fields, methods and protocols. I think the missing pieces are an easier way to convert these classes into a PyErr, so that they can be raised to Python and a way to make that work on abi3 (potentially #4678 can help with that)