Configure tox to use system Python

This makes it usable by contributors by default. CI pipelines explicitly
override the version of Python used.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = {py38,py39,py310,py311}{,-images}
+envlist = py,py-images
 skip_missing_interpreters = True
 
 [testenv]