update datetime.utcnow() to use UTC timezone-aware object

[lebaudantoine]

Following deprecation warning was raised:

  /usr/local/lib/python3.12/site-packages/livekit/api/access_token.py:152: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    (datetime.datetime.utcnow() + self.ttl).utctimetuple()

datetime.utcnow() is being deprecated in Python 3.12 as it returns a naive datetime object without timezone information. Replace with datetime.now(UTC) to use an explicit timezone-aware object, preventing potential timezone ambiguity issues.

In access_token.py, updated the token expiration calculation to use the recommended timezone-aware approach.

[davidzhao]

lgtm

[davidzhao]

thanks for the fix!