Issue with sig.read_annotconfig in cell_type

Hi all, I've encounterd an issue with the **workbooks/celltype_annotation_besca.ipynb** notebook. Running the `sigconfig, levsk = bc.tl.sig.read_annotconfig(configfile)` chunk returns an empty `levsk` object.
```
-----
anndata 0.10.7
besca 2.5.3
matplotlib 3.7.1
numpy 1.26.4
pandas 2.2.2
scanpy 1.10.1
scipy 1.13.0
seaborn 0.13.2
sinfo 0.3.4
-----
IPython 8.18.1
jupyter_client 8.6.1
jupyter_core 5.7.2
jupyterlab 4.1.6
notebook 7.1.2
-----
Python 3.9.19
```

Many thanks!

Start with workbooks/celltype_annotation_besca.ipynb and run the sig.read_annotconfig(configfile) chunk using the reported environment. Trace why the returned levsk object is empty, then verify that the notebook produces the expected non-empty result.

Written by the indexing model from the issue text.