[python][sklearn] add __sklearn_is_fitted__() method to be better c…

…ompatible with scikit-learn API (#4636)

python-package/lightgbm/sklearn.py

@@ -529,6 +529,9 @@ def _more_tags(self):
             }
         }
 
+    def __sklearn_is_fitted__(self) -> bool:
+        return getattr(self, "fitted_", False)
+
     def get_params(self, deep=True):
         """Get parameters for this estimator.