REF: raise scalar_data_error to make mypy happy
@simonjayhawkins do I need to update a config file or enable something?
Have you tried using `NoReturn` for the return value of `_scalar_data_error`? It wasn't added until Python 3.5.4 so will fail the CI pinned to 3.5.3, but could be reasonable to use going into 1.0.0 I haven't tried that, will look into it for future reference.  In this particular case I agree with mypy that this is clearer than the status quo > In this particular case I agree with mypy that this is clearer than the status quo

tend to agree, although the `NoReturn` is intended for patterns like this.  lgtm. @WillAyd Thanks!