MAINT Python 3.9 in badge on README.rst (scikit-learn#19702)

Author: ogrisel

README.rst

@@ -17,8 +17,8 @@
 .. |Nightly wheels| image:: https://github.com/scikit-learn/scikit-learn/workflows/Wheel%20builder/badge.svg?event=schedule
 .. _`Nightly wheels`: https://github.com/scikit-learn/scikit-learn/actions?query=workflow%3A%22Wheel+builder%22+event%3Aschedule
 
-.. |PythonVersion| image:: https://img.shields.io/badge/python-3.6%20%7C%203.7%20%7C%203.8-blue
-.. _PythonVersion: https://img.shields.io/badge/python-3.6%20%7C%203.7%20%7C%203.8-blue
+.. |PythonVersion| image:: https://img.shields.io/badge/python-3.6%20%7C%203.7%20%7C%203.8%20%7C%203.9-blue
+.. _PythonVersion: https://img.shields.io/badge/python-3.6%20%7C%203.7%20%7C%203.8%20%7C%203.9-blue
 
 .. |PyPi| image:: https://badge.fury.io/py/scikit-learn.svg
 .. _PyPi: https://badge.fury.io/py/scikit-learn