iotests/pylintrc: allow up to 10 similar lines

iotests/pylintrc: allow up to 10 similar lines

We want to have similar QMP objects in different tests. Reworking these
objects to make common parts by calling some helper functions doesn't
seem good. It's a lot more comfortable to see the whole QAPI request in
one place.

So, let's increase the limit, to unblock further commit
"iotests: add backup-discard-source"

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>