bpo-37191: the new vector APIs declare "static inline" functions which are no C89 compatible and so cause compilation issues on pygobject3, python-dbus, xen (for example).

I propose to move the new *private* declarations to the internal C API.

I started to work on an application. The blocker issue is _PyObject_CallNoArg() which is now commonly used in CPython code base for best performances. It is used the _testcapi which must *not* be compiled with the internal C API.

So I suggest to first add a new public PyObject_CallNoArg() function. It would be different than _PyObject_CallNoArg() static inline function: PyObject_CallNoArg() would be a regular function and so fit better with ABI issues.