text: Title: Add 'sklearn.ensemble._stacking.StackingClassifier'
  Body: A user requested us to add `sklearn.ensemble._stacking.StackingClassifier`.  (See [doc](https://scikit-learn.org/stable/modules/generated/sklearn.ensemble.StackingClassifier.html)).  
  
  At first glance this appears it should be reasonably easy to implement/similar to our other classifier converters.