Other Python implementations have to provide a compatible C API, as complete and compatible as possible. The problem is that the C API leaks many implementation details like memory layout (structures like PyTupleObject or PyDictObject), memory allocation (objects must be allocated on the heap), reference counting, etc. The C API doesn't fit well with a moving GC for example.

While some core devs tries to reach consumers of this API and developers of other Python implementations, in practice, CPython still has the lead the C API and dictates how other Python implementations must provide a C API.

Maybe the HPy project will change that, since HPy gives a great freedom on how a Python implementation supports HPy. In the meanwhile, there is a long list of C extensions accessing directly the C API.

Other option is to migrate existing C extensions to higher level API like Cython, HPy, cffi, pybind11 or anything else: no longer access the C API directly.