Deprecate / remove `_Py` internal APIs from `pyo3-ffi`

The main outcome I got from my stream today was that @encukou and @vstinner (and presumably the rest of the Core Devs) want to regard CPython C API symbols prefixed with _Py as internal APIs which don't carry any version stability (not even necessarily in the same Python minor version patch series).

I think this means that we should consider removing all such definitions with this _Py prefix from pyo3-ffi, or at the very least making users think very hard before they choose to use these, if there's a good reason for any of these to be exposed in PyO3.

I took some notes on stream about where we use these definitions in PyO3 itself, they're on my desktop PC along with a small patch which removes use of a couple, I'll aim to push both tomorrow.

Start by inventorying the _Py definitions in pyo3-ffi and the places PyO3 uses them, then review the linked CPython C API context. The scope and completion criteria still need a decision: remove every definition, or retain selected APIs with an explicit warning and justification.