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

--- natsort-8.3.1/tox.ini.orig
+++ natsort-8.3.1/tox.ini
@@ -31,9 +31,9 @@
     # 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 --cov {envsitepackagesdir}/natsort --cov-report term-missing {posargs:}
+    pytest --hypothesis-profile=slow-tests --tb=short --cov {env:PYTHONPATH}/natsort --cov-report term-missing {posargs:}
 
 # Check code quality.
 [testenv:flake8]