Fix for Python 4: replace unsafe six.PY3 with PY2
Fix for Python 4: replace unsafe six.PY3 with PY2

pradyunsg/pip,xavfernandez/pip,xavfernandez/pip,pfmoore/pip,pypa/pip,sbidoul/pip,xavfernandez/pip,pfmoore/pip,sbidoul/pip,rouge8/pip,pypa/pip,rouge8/pip,rouge8/pip,pradyunsg/pip