Lists names of columns that have required fields.

def get_required_columns(self):
        """Lists names of columns that have required fields."""
        required_columns = []
        if self.formset_class:
            empty_form = self.get_formset().empty_form
            for column in self.columns.values():
                field = empty_form.fields.get(column.name)
                if field and field.required:
                    required_columns.append(column.name)
        return required_columns