ENH: PEP 803 support

[neutrinoceros]

PEP 803 was accepted. This means Python 3.15t will get support for limited-API/stable ABI.
I'll need to adapt CPythonLimitedAPI to reflect this. Crucially, I might need to introduce finer CPythonABI3 and CPythonABI3T classes.

[neutrinoceros]

Thinking a bit more about this, I'm not sure the Feature interface is even suited to describe compatible ABIs ...
I should probably avoid conflating these completely and instead start from scratch.

[neutrinoceros]

I think the functionality I want would more naturally live in https://packaging.pypa.io/en/stable/tags.html#packaging.tags.cpython_tags
and there's already a PR on its way there pypa/packaging#1099