Fix and reenable useless-object-inheritance.

Initial change generated using:
git ls-tree -r HEAD | awk '$1 != "120000" {print $NF}' | \
  grep '\.py$' | grep -v -e ^third_party/ -e '_pb2\.py' | \
  xargs sed -i 's/(object)//g'

Required only a handful of manual edits after that, mostly for
unrelated lint fixes.

BUG=None
TEST=run_tests, CQ

Change-Id: I0c57fdcf20124530ecd33ae3118506a2be9fc0ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4637436
Auto-Submit: Alex Klein <saklein@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
diff --git a/scripts/sysmon/mainlib.py b/scripts/sysmon/mainlib.py
index e23a9ac..fd9e040 100644
--- a/scripts/sysmon/mainlib.py
+++ b/scripts/sysmon/mainlib.py
@@ -26,7 +26,7 @@
 logger = logging.getLogger(__name__)
 
 
-class _MetricCollector(object):
+class _MetricCollector:
     """Metric collector class."""
 
     def __init__(self):
@@ -50,7 +50,7 @@
         metrics.Flush()
 
 
-class _TimedCallback(object):
+class _TimedCallback:
     """Limits callback to one call in a given interval."""
 
     def __init__(self, callback, interval):