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/cros_mark_as_stable_unittest.py b/scripts/cros_mark_as_stable_unittest.py
index 57b1840..79f9a7f 100644
--- a/scripts/cros_mark_as_stable_unittest.py
+++ b/scripts/cros_mark_as_stable_unittest.py
@@ -121,7 +121,7 @@
         self.assertRaises(AssertionError, self._TestPushChange, bad_cls=True)
 
 
-class EbuildMock(object):
+class EbuildMock:
     """Mock portage_util.Ebuild."""
 
     def __init__(self, path, new_package=True):