Use Python 3.8 for standalone installations instead of 3.10

This sidesteps issues with the relatively-new files() / Traversable API
of importlib.resources that is present in 3.9 and 3.10 but not supported
by PyOxidizer's OxidizedImporter.  It only supports the original
importlib.resources API and importlib.resources doesn't implement a
backwards compatibility layer/adapter until Python 3.11 (not yet
released).  By using 3.8, code which tests for the presence of the new
API in importlib.resources or tests the Python version will not detect
stdlib support for the new API and thus will generally fall back to the
API supported by PyOxidizer.

Related-to: <certifi/python-certifi#203>

Author: tsibley

pyoxidizer.bzl

@@ -24,7 +24,7 @@ def make_exe():
     # Obtain the default PythonDistribution for our build target.  We link this
     # distribution into our produced executable and extract the Python standard
     # library from it.
-    python_dist = default_python_distribution()
+    python_dist = default_python_distribution(python_version = "3.8")
 
     # This function creates a `PythonPackagingPolicy` instance, which
     # influences how executables are built and how resources are added to the