Properties on exception types

[davidcole1340]

Hello! First of all, thanks for the great work on pybind11! It's made writing Python/C bindings very easy 💯

My module exposes a C++ exception using py::register_exception(). On the C++ side, this exception type contains an error code that I want to be accessible from Python-land.

Is it currently possible to expose this error code to Python? I've had a look but seems this is not the case.

Checking the CPython docs, it seems the PyErr_NewException takes a dict argument that could be used to expose this property, but it seems pybind doesn't allow setting this dict argument when it creates an exception. Perhaps this could be used?

[davidcole1340]

Actually this is a duplicate of #5275