fix: datetime serialisation

[mxschmitt]

Motivation:

  /Users/maxschmitt/Developer/playwright-python/tests/sync/test_assertions.py:166: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    {"a": 1, "b": "string", "c": datetime.utcfromtimestamp(1627503992000 / 1000)},