add pragma no cover to not covered line

Author: Carreau

zarr/indexing.py

@@ -878,7 +878,7 @@ def __init__(self, selection, arr_shape):
         self.selection_shape = np.empty(self.arr_shape)[self.selection].shape
         if any([selection_dim < 0 or selection_dim > arr_dim for selection_dim, arr_dim
                 in zip(self.selection_shape, self.arr_shape)]):
-            raise IndexError('a selection index is out of range for the dimension')
+            raise IndexError('a selection index is out of range for the dimension') # pragma: no cover
 
         for i, dim_size in enumerate(self.arr_shape[::-1]):
             index = len(self.arr_shape) - (i+1)