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/api/field_handler.py b/api/field_handler.py
index 6079f87..80a1da5 100644
--- a/api/field_handler.py
+++ b/api/field_handler.py
@@ -31,7 +31,7 @@
     """Result path is invalid."""
 
 
-class ChrootHandler(object):
+class ChrootHandler:
     """Translate a Chroot message to chroot enter arguments and env."""
 
     def __init__(self, clear_field):
@@ -106,7 +106,7 @@
     return None
 
 
-class PathHandler(object):
+class PathHandler:
     """Handles copying a file or directory into or out of the chroot."""
 
     INSIDE = common_pb2.Path.INSIDE
@@ -210,7 +210,7 @@
             self.field.CopyFrom(self._original_message)
 
 
-class SyncedDirHandler(object):
+class SyncedDirHandler:
     """Handler for syncing directories across the chroot boundary."""
 
     def __init__(