Update versions.rst

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: corneliusroemer

versions.rst

@@ -5,7 +5,7 @@
 Status of Python versions
 =========================
 
-The ``main`` branch is currently the future Python |mainversion|, and is the only
+The ``main`` branch is currently the future Python |main_version|, 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/>`_.