Add quotes to numeric answers

[dslavicek]

I was unable to run the quiz when numbers were not quoted, because they were interpreted as numbers and violated expected format somewhere.

[ZelenyMartin]

Thanks for the contribution with the fix! Unfortunately I'd go different way and allow users to use also non-string types in the answer field in YAML. Even that it semantically doesn't make sense to allow ints, floats and bools, we shouldn't limit it for the simplicity of creating a quiz in YAML.

I suggest that the Pydantic model should allow str | int | float | bool for this field, but we should immediately convert it to str.