[Typing] `list_tasks` can be made more explicit in return type with `@overload`

[eddiebergman]

By using an @overload, on output_type: Literal["dict", "dataframe"] this can be converted used to give the correct type returned.

https://github.com/openml/openml-python/blob/56895c252dccb0ef9550f73b1694c62965b85abd/openml/tasks/functions.py#L129-L136

• Could be partially done for get_X_and_y, at least for dataframe.