pyoxidizer: pin Python to 3.8
Newer versions of PyOxidizer use version 3.9 by default. As part of
upgrading PyOxidizer, we want to pin the version at 3.8 so we can
compare differences more easily.
Differential Revision: https://phab.mercurial-scm.org/D10679
pyoxidizer: remove some boilerplate in file
This isn't used for anything and can safely be removed.
Differential Revision: https://phab.mercurial-scm.org/D10678