stop using deprecated datetime.utcnow()

Deprecated since python 3.12. The suggested API seems as having been
available since 3.2 at least.

pytest_docker_compose/plugin.py

@@ -1,6 +1,6 @@
 import os.path
 import warnings
-from datetime import datetime
+from datetime import datetime, timezone
 from pathlib import Path
 
 import pytest
@@ -222,7 +222,7 @@ def generate_scoped_containers_fixture(cls, scope: str):
 
         @pytest.fixture(scope=scope)  # type: ignore
         def scoped_containers_fixture(docker_project: DockerClient, request):
-            now = datetime.utcnow()
+            now = datetime.now(timezone.utc)
             if not request.config.getoption("--use-running-containers"):
                 if any(
                     (