Python 3.13: `src/mailmanclient/tests/test_list.py::TestHeldMessage::test_held_message_moderation` hangs

Hi 👋

We're currently upgrading to Python 3.13 on Arch Linux. This project seems to run into issues with the new interpreter version though.

The test src/mailmanclient/tests/test_list.py::TestHeldMessage::test_held_message_moderation hangs forever.