fix: datetime.utcnow() deprecation warning

[nautics889]

Taken measures according to warning: https://docs.python.org/3/library/datetime.html#datetime.datetime.utcnow
Does not affect any logic and behavior.

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• (fix): precaution of .utcnow() is deprecated

[nautics889]

It seems that check failures above are not caused by this commit itself.

Logs from build 3.12

return self.parse_json(res.text)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/pixivpy/pixivpy/pixivpy3/api.py", line 49, in parse_json
    return json.loads(json_str, object_hook=JsonDict)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.2/x64/lib/python3.12/json/__init__.py", line 359, in loads
>>> 海沿いの街, origin url: https://i.pximg.net/c/600x1200_90/img-master/img/20[16](https://github.com/upbit/pixivpy/actions/runs/8583558571/job/23522908464?pr=348#step:7:17)/10/22/10/11/37/59580629_p0_master1200.jpg
    return cls(**kw).decode(s)
           ^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.2/x64/lib/python3.12/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.2/x64/lib/python3.12/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/pixivpy/pixivpy/demo.py", line 343, in <module>
    main()
  File "/home/runner/work/pixivpy/pixivpy/demo.py", line 330, in main
    appapi_illust(aapi)
  File "/home/runner/work/pixivpy/pixivpy/demo.py", line 34, in appapi_illust
    json_result = aapi.illust_comments(59580629)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/pixivpy/pixivpy/pixivpy3/aapi.py", line [28](https://github.com/upbit/pixivpy/actions/runs/8583558571/job/23522908464?pr=348#step:7:29)8, in illust_comments
    return self.parse_result(r)
           ^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/pixivpy/pixivpy/pixivpy3/aapi.py", line 98, in parse_result
    raise PixivError("parse_json() error: %s" % e, header=res.headers, body=res.text)
pixivpy3.utils.PixivError: parse_json() error: Expecting value: line 1 column 1 (char 0)
Error: Process completed with exit code 1.

And that pixivpy3.utils.PixivError: parse_json() error happens because here in illust_comments() it goes down with 404 Response for request to the endpoint app-api.pixiv.net/v1/illust/comments.

And the same happens when switch to latest release of PixivPy (3.7.5), so I bet it's unrelated to changes in this commit.

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I've created a separate issue for that: #349
I guess it'd be good to hold this PR for a while, before solving that issue.