I don't want to add many functions to limited/public APIs.
Note that public APIs defined outside of cpython/ are considered mandatory for all Python implementations supporting Python/C API.

When portability is more important than performance, there are many APIs already.
Please keep _PyObject_CallOneArg() as just an easy wrapper of _PyObject_Vectorcall which is CPython specific in this issue.

Candidate of revert: _csv, _decimal, _sqlite, pyexpat, _testbuffer, _xxtestfuzz
They doesn't use much private APIs, or they are tests which is useful for other interpreters.