Deal with the case when `.attrs`/`.uns` are not JSON-serializable

[LucaMarconato]

See context here: #data-structures > JSON in `uns`.

Status:

• .attrs in pandas (hence in geopandas and dask.dataframe) are JSON-serializable, as shown here.
• In SpatialData we can enforce that.
• In xarray this needs to be investigated.
• In AnnData this will be the case only if a certain option is toggled, see discussion here#data-structures > JSON in `uns` @ 💬.