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/_operator.c

@@ -1682,7 +1682,7 @@ methodcaller_reduce(methodcallerobject *mc, PyObject *Py_UNUSED(ignored))
 
         newargs[0] = (PyObject *)Py_TYPE(mc);
         newargs[1] = mc->name;
-        constructor = _PyObject_FastCallDict(partial, newargs, 2, mc->kwds);
+        constructor = PyObject_VectorcallDict(partial, newargs, 2, mc->kwds);
 
         Py_DECREF(partial);
         return Py_BuildValue("NO", constructor, mc->args);