Rely less on PyRef

[cjdoris]

Originally PyRef was intended to be the single place to safely hold a reference to a Python object, so that other wrapper types (PyObject, PyList, etc.) could each simply have a PyRef field.

It would actually be simpler to just have these types hold a PyPtr and have a generic finalizer that decrefs whatever is in the ptr field (i.e. re-use pyref_finalize!).