increase scikit-learn to current 0.21.3

requirements_dev.txt

@@ -44,7 +44,7 @@ python-dateutil>=2.7.3
 pytz>=2018.5
 pyzmq>=17.1.2
 qtconsole>=4.4.1
-scikit-learn>=0.21.0
+scikit-learn>=0.21.3
 scipy>=1.1.0
 seaborn>=0.9.0
 Send2Trash>=1.5.0