iotests: Update for pylint 2.11.1

iotests: Update for pylint 2.11.1

1. Ignore the new f-strings warning, we're not interested in doing a
   full conversion at this time.

2. Just mute the unbalanced-tuple-unpacking warning, it's not a real
   error in this case and muting the dozens of callsites is just not
   worth it.

3. Add encodings to read_text().

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210923180715.4168522-7-jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>