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.
Windows .msi and .exe installers now use Python 3.9 instead of Python 3.8.
Differential Revision: https://phab.mercurial-scm.org/D10689