Replace python-igraph requirement with igraph >= 0.9.8 (giotto-ai#616)

python-igraph is deprecated as described in https://pypi.org/project/python-igraph/

Author: ulupo

requirements.txt

@@ -4,6 +4,6 @@ joblib >= 0.16.0
 scikit-learn >= 0.23.1
 giotto-ph >= 0.2.1
 pyflagser >= 0.4.3
-python-igraph >= 0.8.2
+igraph >= 0.9.8
 plotly >= 4.8.2
 ipywidgets >= 7.5.1