Issue parsing a single instance of TagRemovePreprocessor

[Nicolas-CF]

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