Revert "test: fix deprecated datetime serialisation (follow-up to #2314)"

This reverts commit 51bab39.

Author: mxschmitt

tests/async/test_assertions.py

@@ -13,8 +13,8 @@
 # limitations under the License.
 
 import asyncio
-import datetime
 import re
+from datetime import datetime
 
 import pytest
 
@@ -183,11 +183,7 @@ async def test_assertions_locator_to_have_js_property(
     )
     await expect(page.locator("div")).to_have_js_property(
         "foo",
-        {
-            "a": 1,
-            "b": "string",
-            "c": datetime.datetime.fromtimestamp(1627503992000 / 1000),
-        },
+        {"a": 1, "b": "string", "c": datetime.utcfromtimestamp(1627503992000 / 1000)},
     )