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
diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index ddd17a2..6f55577 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -461,7 +461,7 @@
 buffered_simple_flush(buffered *self, PyObject *args)
 {
     CHECK_INITIALIZED(self)
-    return _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_flush);
+    return PyObject_CallMethodNoArgs(self->raw, _PyIO_str_flush);
 }
 
 static int
@@ -513,7 +513,7 @@
     }
     /* flush() will most probably re-take the lock, so drop it first */
     LEAVE_BUFFERED(self)
-    res = _PyObject_CallMethodNoArgs((PyObject *)self, _PyIO_str_flush);
+    res = PyObject_CallMethodNoArgs((PyObject *)self, _PyIO_str_flush);
     if (!ENTER_BUFFERED(self))
         return NULL;
     if (res == NULL)
@@ -521,7 +521,7 @@
     else
         Py_DECREF(res);
 
-    res = _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_close);
+    res = PyObject_CallMethodNoArgs(self->raw, _PyIO_str_close);
 
     if (self->buffer) {
         PyMem_Free(self->buffer);
@@ -545,7 +545,7 @@
 {
     PyObject *raw, *res;
     CHECK_INITIALIZED(self)
-    res = _PyObject_CallMethodNoArgs((PyObject *)self, _PyIO_str_flush);
+    res = PyObject_CallMethodNoArgs((PyObject *)self, _PyIO_str_flush);
     if (res == NULL)
         return NULL;
     Py_DECREF(res);
@@ -562,21 +562,21 @@
 buffered_seekable(buffered *self, PyObject *Py_UNUSED(ignored))
 {
     CHECK_INITIALIZED(self)
-    return _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_seekable);
+    return PyObject_CallMethodNoArgs(self->raw, _PyIO_str_seekable);
 }
 
 static PyObject *
 buffered_readable(buffered *self, PyObject *Py_UNUSED(ignored))
 {
     CHECK_INITIALIZED(self)
-    return _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_readable);
+    return PyObject_CallMethodNoArgs(self->raw, _PyIO_str_readable);
 }
 
 static PyObject *
 buffered_writable(buffered *self, PyObject *Py_UNUSED(ignored))
 {
     CHECK_INITIALIZED(self)
-    return _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_writable);
+    return PyObject_CallMethodNoArgs(self->raw, _PyIO_str_writable);
 }
 
 static PyObject *
@@ -599,14 +599,14 @@
 buffered_fileno(buffered *self, PyObject *Py_UNUSED(ignored))
 {
     CHECK_INITIALIZED(self)
-    return _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_fileno);
+    return PyObject_CallMethodNoArgs(self->raw, _PyIO_str_fileno);
 }
 
 static PyObject *
 buffered_isatty(buffered *self, PyObject *Py_UNUSED(ignored))
 {
     CHECK_INITIALIZED(self)
-    return _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_isatty);
+    return PyObject_CallMethodNoArgs(self->raw, _PyIO_str_isatty);
 }
 
 /* Forward decls */
@@ -670,7 +670,7 @@
 {
     Py_off_t n;
     PyObject *res;
-    res = _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_tell);
+    res = PyObject_CallMethodNoArgs(self->raw, _PyIO_str_tell);
     if (res == NULL)
         return -1;
     n = PyNumber_AsOff_t(res, PyExc_ValueError);
@@ -1324,7 +1324,7 @@
             goto end;
         Py_CLEAR(res);
     }
-    res = _PyObject_CallMethodOneArg(self->raw, _PyIO_str_truncate, pos);
+    res = PyObject_CallMethodOneArg(self->raw, _PyIO_str_truncate, pos);
     if (res == NULL)
         goto end;
     /* Reset cached position */
@@ -1351,7 +1351,7 @@
         line = _buffered_readline(self, -1);
     }
     else {
-        line = _PyObject_CallMethodNoArgs((PyObject *)self,
+        line = PyObject_CallMethodNoArgs((PyObject *)self,
                                              _PyIO_str_readline);
         if (line && !PyBytes_Check(line)) {
             PyErr_Format(PyExc_OSError,
@@ -1470,7 +1470,7 @@
        raised (see issue #10956).
     */
     do {
-        res = _PyObject_CallMethodOneArg(self->raw, _PyIO_str_readinto, memobj);
+        res = PyObject_CallMethodOneArg(self->raw, _PyIO_str_readinto, memobj);
     } while (res == NULL && _PyIO_trap_eintr());
     Py_DECREF(memobj);
     if (res == NULL)
@@ -1569,7 +1569,7 @@
         }
 
         /* Read until EOF or until read() would block. */
-        data = _PyObject_CallMethodNoArgs(self->raw, _PyIO_str_read);
+        data = PyObject_CallMethodNoArgs(self->raw, _PyIO_str_read);
         if (data == NULL)
             goto cleanup;
         if (data != Py_None && !PyBytes_Check(data)) {
@@ -1818,7 +1818,7 @@
     */
     do {
         errno = 0;
-        res = _PyObject_CallMethodOneArg(self->raw, _PyIO_str_write, memobj);
+        res = PyObject_CallMethodOneArg(self->raw, _PyIO_str_write, memobj);
         errnum = errno;
     } while (res == NULL && _PyIO_trap_eintr());
     Py_DECREF(memobj);