Fix type error

[vthemelis]

This is to pacify pyre or any other types checker that someone might be using in conjunction to this project.

[mariobuikhuizen]

I think we want to use the same linting setup as our other projects (e.g., https://github.com/widgetti/solara) here. We should check it with pre-commit and in CI. However, we still have an issue with running it in CI (see widgetti/solara#225). Otherwise, we'll have no way of knowing whether this fixes the lint error. Additionally, we'll keep getting new code with potential lint errors.

[vthemelis]

Fair. I normally use pyre for type checking. I think it's slightly nicer to work with in codebase with sparse initial type annotations and it's also much faster. I see that in solar you're using mypy. Would you like me to stick with that?