python: always assign best_score after train (#890)

This commit will regain backward compatibility that has been broken
after 015c8fff.

python-package/lightgbm/sklearn.py

@@ -452,6 +452,7 @@ class LGBMModel(_LGBMModelBase):
 
         if early_stopping_rounds is not None:
             self._best_iteration = self._Booster.best_iteration
+
         self._best_score = self._Booster.best_score
 
         # free dataset