schedgraph.py: port to Python 3

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Summary

The change does not preserve compatibility with Python 2.7.

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I used to program in Python many years ago, but I am not very good with it now.
This is my attempt to make the script work with Python 3 as it is the default Python now.

It may be worthwhile fixing the code style in the script to conform to PEP8, etc just to appease linters, code editors, etc.

[jmg]

tools/sched/schedgraph.py

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Looks like you might want to add @total_ordering to class EventSource
from functools. For more information:
https://portingguide.readthedocs.io/en/latest/comparisons.html

and the decorator definition:
https://docs.python.org/3/library/functools.html#functools.total_ordering

This will complete the port from single func cmp to the new Python 3 method.

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per jmg's suggestion add total_ordering decorator to get all
comparison operators