freeze scikit-learn versions to 0.20.3

because posterior versions are not supported in 3.4

ippisite/requirements-core.txt

@@ -13,7 +13,7 @@ bioservices
 bioblend
 bs4
 # libraries to compute PCA
-sklearn
+scikit-learn==0.20.3 #later versions do not support Python<3.5
 scipy==1.2.1 #Scipy >=1.3 does not support Python<3.5
 matplotlib==2.2.3 #Matplotlib 3.0+ does not support Python 2.x, 3.0, 3.1, 3.2, 3.3, or 3.4.
 # postgres driver