Revert to using direct import for the six module

Author: MarcCote

nibabel/streamlines/tests/test_trk.py

@@ -5,7 +5,7 @@
 import numpy as np
 from os.path import join as pjoin
 
-from nibabel.externals.six import BytesIO
+from six import BytesIO
 
 from nibabel.testing import data_path
 from nibabel.testing import clear_and_catch_warnings, assert_arr_dict_equal