Using the @singleton errors on default metaclass.

[dangle]

When using @singleton on a class that does not inherit from Object (no other metaclasses defined) throws an exception.

Traceback (most recent call last):
  File "<input>", line 1, in <module>
    @singleton
  File "/home/dangle/Projects/contains.io/containment/.tox/py36/lib/python3.6/site-packages/typet/meta.py", line 58, in _inner
    class _Meta(base, _Meta):  # pylint: disable=function-redefined
TypeError: Cannot create a consistent method resolution
order (MRO) for bases type, Singleton