pyoxidizer: use Python 3.9 (BC)

Newer versions of PyOxidizer use Python 3.9 by default. We
previously pinned the version to 3.8 to facilitate porting to
a new PyOxidizer version and diffing results.

Now that the porting work is complete, let's bump Python
to Python 3.9.

This will effectively change our Windows Inno and WiX Python 3
installers from Python 3.8 to 3.9.

.. bc::

   Windows .msi and .exe installers now use Python 3.9 instead of
   Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D10689

rust/hgcli/pyoxidizer.bzl

@@ -40,7 +40,7 b' RUN_CODE = "import hgdemandimport; hgdem'
 set_build_path(ROOT + "/build/pyoxidizer")
 
 def make_distribution():
-    return default_python_distribution(python_version = "3.8")
+    return default_python_distribution(python_version = "3.9")
 
 def resource_callback(policy, resource):
     if not IS_WINDOWS: