Your patch leaks references with subinterpreters, which was exposed by functools.singledispatch using weakref. While the actual bug is in atexit (which doesn't properly unregister on subinterpreter termination), now all uses of weakref leak.

Context: http://mail.python.org/pipermail/python-dev/2013-June/126755.html

PJE suggests importing atexit and registering finalize only when it's actually used. I guess this would be the easiest workaround.