Deprecate attributes attribute

[Thetwam]

CanvasObject.set_attributes() sets an attribute called attributes that includes all the attributes (as a dict) sent to the function. This is not used anywhere in the function, or anywhere else in the library except for the to_json function.

Based on its docstring, the intention of to_json is to return the original JSON that created the object. As we've discovered (see #303), this is not actually the case. self.attributes can be created, mutated, and updated in many different ways that to_json does not accurately reflect. This information can be gathered from Requester._cache much more reliably. I also have a suspicion that most people don't really use to_json anyways.

We should deprecate attributes and to_json(), and ultimately remove them in a later release.

[Thetwam]

Related: #387