sklearn: Allow 'polars' in set_output (#394)

Author: FBruzzesi

stubs/sklearn/utils/_set_output.pyi

@@ -4,4 +4,4 @@ from ..base import BaseEstimator
 
 class _SetOutputMixin:
     def __init_subclass__(cls, auto_wrap_output_keys: None | tuple[str] = ..., **kwargs) -> None: ...
-    def set_output(self, *, transform: None | Literal["default", "pandas"] = None) -> BaseEstimator: ...
+    def set_output(self, *, transform: None | Literal["default", "pandas", "polars"] = None) -> BaseEstimator: ...