We need to test natsort in proto area.  Currently PYTHONPATH points there.

--- natsort-8.4.0/tox.ini.orig
+++ natsort-8.4.0/tox.ini
@@ -29,7 +29,7 @@
     # Doctests
     {envpython} -m doctest -o IGNORE_EXCEPTION_DETAIL docs/howitworks.rst
     pytest README.rst docs/examples.rst
-    pytest --doctest-modules {envsitepackagesdir}/natsort
+    pytest --doctest-modules {env:PYTHONPATH}/natsort
     # Full test suite. Allow the user to pass command-line objects.
     pytest --hypothesis-profile=slow-tests --tb=short {posargs:}