Issue parsing a single instance of TagRemovePreprocessor

```
pip show traitlets
Name: traitlets
Version: 5.0.5
```
Calling `voila` with the `TagRemovePreprocessor.remove_cell_tags` option fails for a single tag:
```
voila notebook.ipynb --TagRemovePreprocessor.remove_cell_tags tag
....
traitlets.traitlets.TraitError: The 'remove_cell_tags' trait of a TagRemovePreprocessor instance expected a set, not the str 'tag'.
```

But when providing 2 tags, there's no issue:

`voila .\notebook.ipynb --TagRemovePreprocessor.remove_cell_tags tag0 --TagRemovePreprocessor.remove_cell_tags tag1`