unexpected behavior of `datetime.astimezone` method

Bug report

Bug description:

my system time zone is UTC+04:00

>>> import datetime
>>> datetime.datetime(9999, 12, 31).astimezone() # same for .astimezone(datetime.UTC)
Traceback (most recent call last):
  File "<python-input-46>", line 1, in <module>
    datetime.datetime(9999, 12, 31).astimezone()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
ValueError: year must be in 1..9999, not 10000
>>> datetime.datetime(9999, 12, 31, tzinfo=datetime.UTC).astimezone()
datetime.datetime(9999, 12, 31, 4, 0, tzinfo=datetime.timezone(datetime.timedelta(seconds=14400), '+04'))

same behavior for _pydatetime module, but I found that in version 3.11 the behavior is as next:

>>> datetime.datetime(9999, 12, 31).astimezone()
datetime.datetime(9999, 12, 31, 0, 0, tzinfo=datetime.timezone(datetime.timedelta(seconds=14400), '+04'))

EDIT: same behavior for datetime(1, 1, 1):

>>> datetime.datetime(1, 1, 1).astimezone()
Traceback (most recent call last):
  File "<python-input-23>", line 1, in <module>
    datetime.datetime(1, 1, 1).astimezone()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
ValueError: year must be in 1..9999, not 0

CPython versions tested on:

CPython main branch, 3.13, 3.12

Operating systems tested on:

Linux

Linked PRs

• gh-130752

从 datetime.astimezone 入口点开始，在 CPython main 上复现报告的第 1 年和第 9999 年案例，并将其与 Python 3.11 的行为进行比较。继续之前请查看关联的 PR gh-130752；当边界案例不再意外引发错误，并且预期行为已由相关测试覆盖时，工作即告完成。