trace: implement __lt__ for Trace

Python3 no longer supports __cmp__, we need to define __lt__ to make
the objects comparable. This fixes `trace extract` on GDB linked with
python3.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>