We test in proto area not in site-packages.

--- tox-4.23.2/tox.toml.orig
+++ tox-4.23.2/tox.toml
@@ -22,7 +22,7 @@
       "{work_dir}{/}junit.{env_name}.xml",
       "--no-cov-on-fail",
       "--cov",
-      "{env_site_packages_dir}{/}tox",
+      "{env:PYTHONPATH}{/}tox",
       "--cov",
       "{tox_root}{/}tests",
       "--cov-config",