Update plugin example to work with recent changes

[tpoliaw]

Path::into_pyobject now returns a pathlib.Path object which doesn't work
as expected when added to sys.path so revert to string (see #4925).

impl FromPyObject for PyRefMut now returns a custom error type (that
doesn't impl std::error::Error) so map the error back to PyErr to
return it (see #5413).

[tpoliaw]

Should fix #5570

For the errors, I initially tried adding impl Error for the new errors but the lifetimes didn't allow them to be returned from Python::attach. Is it worth adding them anyway?

[danferns]

Thank you for updating the example, I just tried running it and it works without any issues! 😄

[danferns]

For the errors, I initially tried adding impl Error for the new errors but the lifetimes didn't allow them to be returned from Python::attach. Is it worth adding them anyway?

I had found that the cargo errors go away if the return type for main is set to PyResult<()> (without changing anything else).

fn main() -> PyResult<()> {
/// ...

[tpoliaw]

I think that makes sense because it'll do the conversion to PyErr (that I've done manually here) automatically whereas it can't auto convert to Box<...>. I'll leave it up to @davidhewitt whether it's preferred to change the main return type or explicitly convert the errors. Or I suppose adding a type hint to attach could work as well?

[davidhewitt]

Thanks for this! I think mapping the errors is fine for now 👍