RF: use native versions of callable, io classes

Use native versions rather using using versions from 'six'.

Author: matthew-brett

nibabel/benchmarks/bench_load_save.py

@@ -19,7 +19,7 @@
 
 import numpy as np
 
-from six import BytesIO
+from io import BytesIO
 
 from .. import Nifti1Image
 

nibabel/gifti/parse_gifti_fast.py

@@ -12,7 +12,7 @@
 import sys
 import warnings
 import zlib
-from six import StringIO
+from io import StringIO
 from xml.parsers.expat import ExpatError
 
 import numpy as np

nibabel/optpkg.py

@@ -1,7 +1,7 @@
 """ Routines to support optional packages """
 from distutils.version import LooseVersion
 
-from six import string_types, callable
+from six import string_types
 
 try:
     import nose