bpo-39245: Switch to public API for Vectorcall (GH-18460)

The bulk of this patch was generated automatically with:

    for name in \
        PyObject_Vectorcall \
        Py_TPFLAGS_HAVE_VECTORCALL \
        PyObject_VectorcallMethod \
        PyVectorcall_Function \
        PyObject_CallOneArg \
        PyObject_CallMethodNoArgs \
        PyObject_CallMethodOneArg \
    ;
    do
        echo $name
        git grep -lwz _$name | xargs -0 sed -i "s/\b_$name\b/$name/g"
    done

    old=_PyObject_FastCallDict
    new=PyObject_VectorcallDict
    git grep -lwz $old | xargs -0 sed -i "s/\b$old\b/$new/g"

and then cleaned up:

- Revert changes to in docs & news
- Revert changes to backcompat defines in headers
- Nudge misaligned comments

Modules/_struct.c

@@ -2097,7 +2097,7 @@ cache_struct_converter(PyObject *fmt, PyStructObject **ptr)
         return 0;
     }
 
-    s_object = _PyObject_CallOneArg(_structmodulestate_global->PyStructType, fmt);
+    s_object = PyObject_CallOneArg(_structmodulestate_global->PyStructType, fmt);
     if (s_object != NULL) {
         if (PyDict_GET_SIZE(cache) >= MAXCACHE)
             PyDict_Clear(cache);