test_sys refcount test checks that assigning None to a local variable n
increases the references to None by exactly 1.

However sys.settrace is set, then the frame object must be instantiated
to be passed to the trace object.  This increments the reference count
to None again.  Since the locals are not then removed from the frame
object after the sys.settrace call, the number of references remains
increased after the settrace function is exited.

This problem can be avoided by making n a global.