The recent change in #13715 broke mininode tests on systems with Python 3.4.2 (as included in Debian Jessie aka oldstable). This is because the method is_closing is only introduced in Python 3.4.4.

A workaround would be to add a hasattr condition that only checks is_closing if it is actually available - keeping the benefits of #13715 for newer systems while not breaking old systems (basically falling back to the behaviour before #13715 for them). If that's a desired fix, I'm happy to create a PR for it.