[FIX] Remove remaining import from six

Author: oesteban

nipype/pipeline/engine/tests/test_engine.py

@@ -431,7 +431,7 @@ def test_doubleconnect():
 import nipype.pipeline.engine as pe
 import nipype.interfaces.spm as spm
 import os
-from six import StringIO
+from io import StringIO
 from nipype.utils.config import config
 
 config.readfp(StringIO("""