Update future Python version number (python#595)

Co-authored-by: Manuel Jacob <test>

Author: manueljacob

index.rst

@@ -108,7 +108,7 @@ Status of Python branches
 
 .. Remember to update the end-of-life table in devcycle.rst.
 
-The master branch is currently the future Python 3.9, and is the only
+The master branch is currently the future Python 3.10, and is the only
 branch that accepts new features.  The latest release for each Python
 version can be found on the `download page <https://www.python.org/downloads/>`_.