Change PyDict::items() to return `Vec<(PyObject, PyObject)>` instead of `PyList`.
Change PyDict::items() to return `Vec<(PyObject, PyObject)>` instead of `PyList`.

This makes it possible iterate through a PyDict in Rust without
having to unpack tuples.

The PyList version of the method is renamed to `items_list()`.

[breaking-change]

naufraghi/rust-cpython,dgrunwald/rust-cpython,dgrunwald/rust-cpython,naufraghi/rust-cpython,naufraghi/rust-cpython,fivejjs/rust-cpython,tempbottle/rust-cpython,eneas193933/rust-cpython,ehiggs/rust-cpython,novocaine/rust-cpython