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

rust/hgcli/pyoxidizer.bzl

@@ -8,7 +8,7 b' RUN_CODE = "import hgdemandimport; hgdem'
 set_build_path(ROOT + "/build/pyoxidizer")
 
 def make_distribution():
-    return default_python_distribution()
+    return default_python_distribution(python_version = "3.8")
 
 def make_distribution_windows():
     return default_python_distribution(flavor = "standalone_dynamic")