Fix datetime returning UTC

[cdce8p]

Description

datetime.utcfromtimestamp will emit a DeprecationWarning in Python 3.12.

The "official" replacement would be to use fromtimestamp(tz=timezone.utc).replace(tzinfo=None). However in this particular case datetime(1970, 1, 1) seems to be the easier solution.

https://docs.python.org/3.12/library/datetime.html#datetime.datetime.utcfromtimestamp

[simonc56]

Doesn't seem to be the same either.

>>> datetime.datetime.utcfromtimestamp(0) + datetime.timedelta(seconds=1593721093)
datetime.datetime(2020, 7, 2, 20, 18, 13)

>>> datetime.datetime.fromtimestamp(1593721093)
datetime.datetime(2020, 7, 2, 22, 18, 13)

>>> datetime.datetime.fromtimestamp(0) + datetime.timedelta(seconds=1593721093)
datetime.datetime(2020, 7, 2, 21, 18, 13)

[JonnyWong16]

I think that should work now.

[meisnate12]

I've got a user getting an issue that i think may be related to this PR. The plex instance seems to have some weird addedAt dates and causing ValueError: year 30828 is out of range to be thrown. I'm trying to get the user to fix the metadata in their DB, but they mentioned that it was working fine on the latest PMM which uses PlexAPI v4.15.2 but when switched to develop using v4.15.6 he would see this issue. I can open a new issue if needed but thought id post on this PR first.

LibrarySection.findItems(data, initpath=key)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/plexapi/base.py", line 329, in findItems
  item = self._buildItemOrNone(elem, cls, initpath)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/plexapi/base.py", line 109, in _buildItemOrNone
  return self._buildItem(elem, cls, initpath)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/plexapi/base.py", line 101, in _buildItem
  return ecls(self._server, elem, initpath)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/plexapi/base.py", line 62, in __init__
  self._loadData(data)
File "/usr/local/lib/python3.11/site-packages/plexapi/video.py", line 363, in _loadData
  Video._loadData(self, data)
File "/usr/local/lib/python3.11/site-packages/plexapi/video.py", line 51, in _loadData
  self.addedAt = utils.toDatetime(data.attrib.get('addedAt'))
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/plexapi/utils.py", line 331, in toDatetime
  return datetime.fromtimestamp(value)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: year 30828 is out of range