RFC: remove private _Py symbols from pyo3-ffi

[davidhewitt]

Closes #3762

This PR removes (as much as possible) all FFI functions and constants which start with _Py. Given that the guidance in #3762 from the CPython team is that we shouldn't be using these symbols (except in some cases where they are ABI implementation details) I have proceeded to remove them rather than deprecate. In the cases where we need to use them for implementations I removed the pub from the symbol definition so that pyo3-ffi retains use of the symbol without re-exporting.

Marked as draft for now as I need to finish off later; if we agree that this is a good idea then I will:

• Split this out into smaller PRs which are individually reviewable
• Add a migration guide entry to explain to users that some FFI symbols have been removed without deprecation
• Update Architecture.md / Contributing.md to note that we don't expose these symbols.

[alex]

I'm supportive of getting rid of _Py stuff to the maximum extent possible -- they're basically compatibility hazards that reduce the chances a given version of pyo3 will work on a future Python.

I'm even more supportive of splitting these out 😂

To the extent there's stuff we need that doesn't have a public (or abi3) API, we should definitely make sure to turn them all into upstream bug reports.

[codspeed-hq]

CodSpeed Performance Report

Merging #4379 will not alter performance

_{Comparing davidhewitt:remove-private-symbols (11d3f0c) with main (144b199)}

Summary

✅ 73 untouched

[davidhewitt]

Superseded by #5592