Adding PyMethod_New and/or PyMethod_Type to stable ABI / limited API

(Bringing Adding `PyMethod_New` to stable ABI / limited API · Issue #108 · capi-workgroup/decisions · GitHub to Discuss) Projects enabling the implementation of Python modules using foreign (non-C) languages often include an implementation (in C) of some custom Python type which implements tp_call and (often) the vectorcall protocol, and is used to wrap function values of the implementation language in instances and expose the underlying function to Python code. As some examples, nanobind has n...