Fix FutureWarning in python3

Use of the set_dim_info function showed up a FutureWarning error in
python3.   python3 now really dislikes comparing to None using ==,
which is what "in" does.  Work around this to silence the warning.

Signed-off-by: Mark Hymers <[email protected]>

Author: markhymers

nibabel/nifti1.py

@@ -1274,7 +1274,7 @@ def set_dim_info(self, freq=None, phase=None, slice=None):
         This is stored in one byte in the header
         '''
         for inp in (freq, phase, slice):
-            if inp not in (None, 0, 1, 2):
+            if inp is not None and inp not in (0, 1, 2):
                 raise HeaderDataError('Inputs must be in [None, 0, 1, 2]')
         info = 0
         if freq is not None: