text: Title: Mapper - import from inexistent sklearn.cluster._hierarchical
  Body: I wanted to try out giotto mapper  running `Christmas-Mapper.ipynb`,  but when I installed `giotto-learn-nightly` `scikit-learn` was upgraded to 0.22.1, and this new version has changed `sklearn.cluster._hierarchical`  to `sklearn.cluster._hierarchical_fast` (see [https://github.com/scikit-learn/scikit-learn/tree/master/sklearn/cluster](https://github.com/scikit-learn/scikit-learn/tree/master/sklearn/cluster)])
  
  Here giotto.mapper  imports from `sklearn.cluster._hierarchical`:
  [https://github.com/giotto-ai/giotto-learn/blob/master/giotto/mapper/cluster.py#L7](sklearn.cluster._hierarchical).
  
  