Fix multiple inheritance in ComposedSampler and ComposedPolySampler

[randomir]

Currently, Sampler attributes override Composite attributes in ComposedSampler (and similarly in ComposedPolySampler):

dimod/dimod/core/composite.py

Lines 110 to 111 in 7c27eb5

	class ComposedSampler(Sampler, Composite):
	"""Abstract base class for dimod composed samplers.

Up until we implemented Scoped interface (.close()) on both Sampler and Composite, this was not an issue, as their methods didn't overlap.

However, now we want Composite.close() to override Sampler.close(), as it implements a meaningful behavior unlike the no-op close on sampler. Generally as well I think it makes sense for composite attributes to take precedence over sampler attrs on a composed sampler.