python 3.8: `test_date*_behaves_like_date*` tests fail with AttributeError

从 tests/test_items.py 中失败的测试开始，然后检查 tomlkit/items.py 中报告行附近的 DateTime 和 Date 实现。在 Python 3.8 上重现这些失败，并验证 date 和 datetime 行为测试能够通过，同时保留所示的比较和字符串输出。

=================================== FAILURES ===================================
_____________________ test_datetimes_behave_like_datetimes _____________________

    def test_datetimes_behave_like_datetimes():
        i = item(datetime(2018, 7, 22, 12, 34, 56))
    
        assert i == datetime(2018, 7, 22, 12, 34, 56)
        assert i.as_string() == "2018-07-22T12:34:56"
    
>       i += timedelta(days=1)

tests/test_items.py:275: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tomlkit/items.py:525: in __add__
    result = super(DateTime, self).__add__(other)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

cls = <class 'tomlkit.items.DateTime'>, value = 2018
_ = (7, 23, 12, 34, 56, 0, ...)

    def __new__(cls, value, *_):  # type: (..., datetime, ...) -> datetime
        return datetime.__new__(
            cls,
>           value.year,
            value.month,
            value.day,
            value.hour,
            value.minute,
            value.second,
            value.microsecond,
            tzinfo=value.tzinfo,
        )
E       AttributeError: 'int' object has no attribute 'year'

tomlkit/items.py:498: AttributeError
_________________________ test_dates_behave_like_dates _________________________

    def test_dates_behave_like_dates():
        i = item(date(2018, 7, 22))
    
        assert i == date(2018, 7, 22)
        assert i.as_string() == "2018-07-22"
    
>       i += timedelta(days=1)

tests/test_items.py:295: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tomlkit/items.py:584: in __add__
    result = super(Date, self).__add__(other)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

cls = <class 'tomlkit.items.Date'>, value = 2018, _ = (7, 23)

    def __new__(cls, value, *_):  # type: (..., date, ...) -> date
>       return date.__new__(cls, value.year, value.month, value.day)
E       AttributeError: 'int' object has no attribute 'year'

tomlkit/items.py:565: AttributeError