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

Python/errors.c

@@ -93,7 +93,7 @@ _PyErr_CreateException(PyObject *exception, PyObject *value)
         return PyObject_Call(exception, value, NULL);
     }
     else {
-        return _PyObject_CallOneArg(exception, value);
+        return PyObject_CallOneArg(exception, value);
     }
 }
 
@@ -907,7 +907,7 @@ PyErr_SetImportErrorSubclass(PyObject *exception, PyObject *msg,
         goto done;
     }
 
-    error = _PyObject_FastCallDict(exception, &msg, 1, kwargs);
+    error = PyObject_VectorcallDict(exception, &msg, 1, kwargs);
     if (error != NULL) {
         _PyErr_SetObject(tstate, (PyObject *)Py_TYPE(error), error);
         Py_DECREF(error);
@@ -1422,7 +1422,7 @@ _PyErr_WriteUnraisableMsg(const char *err_msg_str, PyObject *obj)
         goto default_hook;
     }
 
-    PyObject *res = _PyObject_CallOneArg(hook, hook_args);
+    PyObject *res = PyObject_CallOneArg(hook, hook_args);
     Py_DECREF(hook_args);
     if (res != NULL) {
         Py_DECREF(res);