FIX polars 0.20.4 deprecation warnings (#133)

* keyword "columns" in drop is deprecated

src/model_diagnostics/scoring/scoring.py

@@ -3,6 +3,7 @@
 Each scoring function is implemented as a class that needs to be instantiated
 before calling the `__call__` methode, e.g. `SquaredError()(y_obs=[1], y_pred=[2])`.
 """
+
 from abc import ABC, abstractmethod
 from typing import Any, Callable, Optional
 
@@ -931,6 +932,6 @@ def decompose(
 
     # Remove column "model" for a single model.
     if n_pred <= 1:
-        df = df.drop(columns="model")
+        df = df.drop("model")
 
     return df