Description of the problem

When using the predict method of any predictor of Verde, sklearn is raising a FutureWarning because the attributes argument on sklearn.utils.validation.check_is_fitted is currently ignored (for sklearn == 0.22) and will be deprecated (on sklearn == 0.23). The function now checks if the predictor has any attribute that ends with a trailing underscore.

Original problem discovered on fatiando/harmonica#133

Possible solution

We would just need to remove the attributes list passed to check_is_fitted function on any predictor from Verde.
We would also need to update any documentation instructing how to subclass BaseGridder.