Simplify class definitions, remove (object)

[stephenworsley]

Simplifies class definitions which only inherit from object.

[bjlittle]

@stephenworsley You have a handful of flake8 violations to address, as highlighted by our ever friendly stickler-ci 😉

[bjlittle]

@stephenworsley Nice.

After a quick search, a few cheeky objects appear to have slipped through your search:

19:class CoordSystem(object, metaclass=ABCMeta):
./coord_systems.py

66:class CFVariable(object, metaclass=ABCMeta):
790:class CFGroup(MutableMapping, object):
./fileformats/cf.py

388:            object.__setattr__(self, name, self[index])
456:                    object.__setattr__(self, name, value)
466:            object.__setattr__(self, name, value)
753:class PPField(object, metaclass=abc.ABCMeta):
./fileformats/pp.py

159:class SymmetricNormalize(mpl_colors.Normalize, object):
./palette.py

3966:class ClassDict(MutableMapping, object):
./cube.py

851:            object.__setattr__(self, name, value)
./util.py

21:class CubeArithmeticBroadcastingTestMixin(object, metaclass=ABCMeta):
110:class CubeArithmeticMaskingTestMixin(object, metaclass=ABCMeta):
185:class CubeArithmeticMaskedConstantTestMixin(object, metaclass=ABCMeta):
./tests/unit/analysis/maths/__init__.py

32:class Mixin_register(object, metaclass=abc.ABCMeta):
./tests/unit/merge/test_ProtoCube.py

17:class _MaskedArrayEquality(object, metaclass=ABCMeta):
./tests/unit/tests/test_IrisTest.py

Fancy following up with these also? 😄

[bjlittle]

Outstanding object reference documented in #3512

[bjlittle]

@stephenworsley LGTM 👍 Thanks!