title: Getting "is_categorical is deprecated" error while using JamesSteinEncoder
body: <p>Getting below error when trying to fit JamesSteinEncoder</p>
  <pre><code>encoder = JamesSteinEncoder().fit(X, y)
  
  FutureWarning: is_categorical is deprecated and will be removed in a future version.  Use is_categorical_dtype instead
    elif pd.api.types.is_categorical(cols):
  
  sklearn.__version__ :  '0.23.2'
  </code></pre>
  <p>Code:</p>
  <pre><code>speed = ['DE','AU','US','FR','GB','KR','AU']
  lifespan = [1, 0, 1, 1, 1, 0, 1]
  life = [1, 1, 0, 1, 1, 0, np.nan]
  index = ['snail', 'pig', 'elephant',
           'rabbit', 'giraffe', 'coyote', 'horse']
  df = pd.DataFrame({'speed': speed,
                     'lifespan': lifespan,
                    'life':life}, index=index)
  
  df['speed']= df['speed'].astype('category')
  
   from category_encoders import JamesSteinEncoder
      X = df['speed']
      y = df['lifespan']
      enc = JamesSteinEncoder().fit(X, y)
  
  /Users/*/opt/anaconda3/envs/proj/lib/python3.7/site-packages/category_encoders/utils.py:21: FutureWarning: is_categorical is deprecated and will be removed in a future version.  Use is_categorical_dtype instead
    elif pd.api.types.is_categorical(cols):
  </code></pre>