Python 3.13: Several blocking tests

Hi 👋

When trying to package postorius for Arch Linux in our Python 3.13 rebuild I ran into four tests never returning:

• src/postorius/tests/mailman_api_tests/test_list_moderation.py::HeldMessageTest::test_accept_held_messages
• src/postorius/tests/mailman_api_tests/test_list_moderation.py::HeldMessageTest::test_moderate_held_message_set_moderation_for_nonmember
• src/postorius/tests/mailman_api_tests/test_list_moderation.py::HeldMessageTest::test_reject_held_message_with_reason
• src/postorius/tests/mailman_api_tests/test_nav_helpers.py::TestNavigationHelpers::test_held_message_count

I am deselecting them now from the test setup, but it would be great to get them working again.