Make `PyObject` parametric?

Thinking about conversion between Julia and Python values and semantics, I wonder if we could make `PyObject` parametric, so make it `PyObject{P <: PyConversionPolicy}` where `P` encodes how we want to interpret the object.

For example, `PyObject{Nice}` would be equivalent to `PyObject` as it is now, with user-friendly automatic conversions. `PyObject{Raw}` would perform no automatic conversion at all, which would be useful for wrapping Python packages into Julia, where complete control is required. Other possibilities could be `PyObject{Numpy}` which converts Julia arrays to numpy arrays instead of to lists, for example.

This policy parameter would be inherited in a natural way, so operations on objects with the same policy produce an object with that policy.

I don't know if the effort to implement this would be worth it, since you can already achieve the same things using PyCall's clunkier syntax, but I just wanted to gather opinions and suggestions.

Start by reading the existing PyObject conversion behavior and the PyCall syntax the issue compares against. Clarify whether a PyConversionPolicy with Nice, Raw, and Numpy variants is wanted, including policy inheritance across operations; done would require agreement on the design and its conversion semantics.

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