Making `PyObject` opaque in the limited API

[alex_Gaynor]

@encukou

I consider building twice OK: I’ve been convinced that stable ABI shouldn’t be about building exactly once, but about building for a range of versions. See cryptography which currently builds for cp37-abi3 and cp39-abi3. Other projects might build a version-specific (optimized) wheel for, say, cp313 and cover the rest with abi3.

Yeah, just to state this explicitly from the PyCA maintainers perspective, as long as we have O(1) builds, that’s ok. What we can’t/won’t do is O(n) where we need new builds for every Python release.

My main piece of substantive feedback is that for those of us working in non-C extension modules (Rust, in our case), API compatibility doesn’t do a ton, so figuring out what the desired ABI story is is incredibly important to us.