Visualisation problem - Can't find the d3 library

[Adahn]

I use Python version 3.7.7.

I tried to visualise the tree of the mushroom_examples you provided :

`from future import print_function
from future import unicode_literals
from future import absolute_import
from future import division
from random import shuffle
from random import seed

import matplotlib.pyplot as plt
from sklearn.decomposition import PCA
from sklearn.feature_extraction import DictVectorizer
from sklearn.metrics import adjusted_rand_score

from concept_formation.cobweb import CobwebTree
from concept_formation.cluster import cluster
from concept_formation.datasets import load_mushroom
from concept_formation.visualize import visualize_clusters

seed(0)
mushrooms = load_mushroom()
shuffle(mushrooms)
mushrooms = mushrooms[:150]

tree = CobwebTree()
mushrooms_no_class = [{a: mushroom[a] for a in mushroom
if a != 'classification'} for mushroom in mushrooms]
clusters = next(cluster(tree, mushrooms_no_class))
visualize_clusters(tree, clusters)`

The page is opening in the browser but the tree is not shown. As you can see on the screenshot, I get the following warning: Can't find the d3 library. Currently the viz assumes it can get it from a CDN on the internet.