Document acceptance of system python 3.12

Co-authored-by: Aliaksei Urbanski <aliaksei.urbanski@gmail.com>

build/pkgs/python3/SPKG.rst

@@ -8,7 +8,7 @@ By default, Sage will try to use system's ``python3`` to set up a virtual
 environment, a.k.a. `venv <https://docs.python.org/3.10/library/venv.html>`_
 rather than building a Python 3 installation from scratch.
 
-Sage will accept versions 3.9.x to 3.11.x.
+Sage will accept versions 3.9.x to 3.12.x.
 
 You can also use ``--with-python=/path/to/python3_binary`` to tell Sage to use
 ``/path/to/python3_binary`` to set up the venv. Note that setting up the venv requires