Add support for setting and retrieving exception cause

[messense]

No description provided.

[davidhewitt]

👍 I'm in favour of adding this. I have a few design choices I want to ask about:

• possibly merge set_cause / clear_cause - see below
• also, these APIs could potentially work with &PyBaseException instead of Self. Would be slightly closer to the underlying C APIs, though might in practice just force users to call .into_instance(py).

I think the answers mostly depend on how you expect users to want to use these APIs? I assume you might have a use case which prompted you to add them.

[messense]

I do have a use case for this, consider that a Rust extension module that accepts and calls a user supplied Python callback, the Python callback can raise many kind of exceptions, but the Rust extension module will wrap them as an unified exception for example RuntimeError, it'd be nice to set the exception cause to whatever exception the Python callback throws so it's easier to debug.

https://github.com/messense/rjsonnet-py/blob/1d57d47863fa47a3bf1bef5562374fa4467731a2/src/lib.rs#L193-L213

[davidhewitt]

👍 thanks for tweaking, I think we can add this!