Update typing for Resample in extra.py - #2323

I believe the typing here should be Optional[List[int]], to match VectorUInt32_Size in the signature for ResampleImageFilter_SetSize.

My IDE has been giving me typing warnings when I provide a List[Int], but the function works as expected. int is actually incorrect:

image = sitk.GetImageFromArray(np.random.random([5, 6, 7]))
resampled_image = sitk.Resample(image, size=[2, 2, 2])  # returns a valid image
resampled_image = sitk.Resample(image, size=[2])  # error
resampled_image = sitk.Resample(image, size=2)  # error