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