flatten arbitrarily nested lists/tuples

# flattens arbitrarily nested sequences
# example: lst = [[1, 2, 3], [7, 3, 4], [2, 5, 6, [11, 12, 13]]]
# returns: [1, 2, 3, 7, 3, 4, 2, 5, 6, 11, 12, 13]

def flatten(items: Sequence, seqtypes=(list, tuple)) -> Sequence:
    """flattens arbitrarily nested lists
    :param :items sequence containing nested items
    :returns flattend result"""

    try:
        for i, x in enumerate(items):
            while isinstance(x, seqtypes):
                items[i:i+1] = x
                x = items[i]
    except IndexError:
        pass
    return items