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/_io/stringio.c

@@ -408,7 +408,7 @@ stringio_iternext(stringio *self)
     }
     else {
         /* XXX is subclassing StringIO really supported? */
-        line = _PyObject_CallMethodNoArgs((PyObject *)self,
+        line = PyObject_CallMethodNoArgs((PyObject *)self,
                                              _PyIO_str_readline);
         if (line && !PyUnicode_Check(line)) {
             PyErr_Format(PyExc_OSError,