Add comparison operator to allow Timers with the same end time to be
pushed into heaps. It's poorly documented, but the warning can be found
here:
https://docs.python.org/2/library/heapq.html#priority-queue-implementation-notes
"In the future with Python 3, tuple comparison breaks for (priority,
task) pairs if the priorities are equal and the tasks do not have a
default comparison order."