gh-136880: Add warning about PYTHONPATH (GH-151098)

Author: glennglazer

Doc/tutorial/venv.rst

@@ -88,6 +88,11 @@ For example:
   '~/envs/tutorial-env/lib/python3.5/site-packages']
   >>>
 
+Note that the activated virtual environment does not alter the ``PYTHONPATH`` variable in any way.
+This may lead to unexpected results if the path includes references to code which is incompatible with
+the Python version the virtual environment is using. The best practice is to ``unset PYTHONPATH``
+in bash or the equivalent for the shell you are using.
+
 To deactivate a virtual environment, type::
 
     deactivate