Use enumerate to iterate over voxels

Co-authored-by: Chris Markiewicz <effigies@gmail.com>

Author: MichielCottaar

nibabel/cifti2/cifti2_axes.py

@@ -776,8 +776,8 @@ def __init__(self, name, voxels, vertices, affine=None, volume_shape=None, nvert
         """
         self.name = np.asanyarray(name, dtype='U')
         self.voxels = np.empty(len(voxels), dtype='object')
-        for idx in range(len(voxels)):
-            self.voxels[idx] = voxels[idx]
+        for idx, vox in enumerate(voxels):
+            self.voxels[idx] = vox
         self.vertices = np.asanyarray(vertices, dtype='object')
         self.affine = np.asanyarray(affine) if affine is not None else None
         self.volume_shape = volume_shape