We were doing this because we use to run tests with `python setup.py test`
which automatically sets PYTHONPATH, so it was affecting sub processes
started by standalone.

Nowadays we no longer use this method of running tests, so we are not really
supposed to use PYTHONPATH, so we don't unset this environment variable,
there might be valid cases where users want to force a different python path
(but I can not think of any) and it simplifies the code a bit.