Pickling error with numba gufunc

I'm trying [this example from the dask-examples repo](https://github.com/dask/dask-examples/blob/main/applications/stencils-with-numba.ipynb). When computing the graph it gives a `PicklingError`
```
_pickle.PicklingError: Can't pickle : it's not the same object as __main__.smooth
```
This happens only when using distributed. I launch a cluster in a Slurm cluster via `ipyparallel`'s `become_dask`. Something like [this](https://ipyparallel.readthedocs.io/en/latest/examples/dask.html). Without using distributed the example works fine.

These are the versions of some relevant packages I'm using:
```
dask-2023.6.0
distributed-2023.6.0
toolz-0.12.0
cloudpickle-2.2.1
msgpack-1.0.5
numba-0.57.0
```

Not sure if this is a bug. Probably it's something in my environment. I found similar serialization errors but nothing recent. Only solved issues earlier than 2022.