python: Fix object comparisons in Python 3.

python: Fix object comparisons in Python 3.

Python 3 no longer supports __cmp__.  Instead, we have to implement the
"rich comparison" operators.  We implement __eq__ and __lt__ and use
functools.total_ordering to implement the rest.

In one case, no __cmp__ method was provided and instead relied on the
default behavior provided in Python 2.  We have to implement the
comparisons explicitly for Python 3.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>