append slice_count to the end of a filename

def _build_sliced_filepath(filename, slice_count):
    """ append slice_count to the end of a filename """
    root = os.path.splitext(filename)[0]
    ext = os.path.splitext(filename)[1]
    new_filepath = ''.join((root, str(slice_count), ext))
    return _build_filepath_for_phantomcss(new_filepath)