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/cbuildbot.py b/scripts/cbuildbot.py
index a2f6f1a..f7c95b9 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -1135,7 +1135,7 @@
     return context
 
 
-class _MockMethodWithReturnValue(object):
+class _MockMethodWithReturnValue:
     """A method mocker which just returns the specific value."""
 
     def __init__(self, return_value):
@@ -1145,7 +1145,7 @@
         return self.return_value
 
 
-class _ObjectMethodPatcher(object):
+class _ObjectMethodPatcher:
     """A simplified mock.object.patch.
 
     It is a context manager that patches an object's method with specified