python 3.12: utcnow function is deprecated

The following warning is displayed on a system running with Python 3.12:
-------------------
/usr/lib/python3.12/site-packages/ipalib/rpc.py:925: DeprecationWarning:
datetime.utcnow() is deprecated and scheduled for removal in a future
version. Use timezone-aware objects to represent datetimes in UTC:
datetime.now(datetime.UTC).

  timestamp=datetime.datetime.utcnow())
-------------------

Fixes: https://pagure.io/freeipa/issue/9425
Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>

ipatests/test_ipapython/test_cookie.py

@@ -402,7 +402,7 @@ class TestHTTPReturn:
             assert cookie.http_return_ok(self.url)
 
     def test_expires(self):
-        now = datetime.datetime.utcnow().replace(microsecond=0)
+        now = datetime.datetime.now(tz=datetime.UTC).replace(microsecond=0)
 
         # expires 1 day from now
         expires = now + datetime.timedelta(days=1)