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/_warnings.c

@@ -593,7 +593,7 @@ call_show_warning(PyObject *category, PyObject *text, PyObject *message,
     if (msg == NULL)
         goto error;
 
-    res = _PyObject_CallOneArg(show_fn, msg);
+    res = PyObject_CallOneArg(show_fn, msg);
     Py_DECREF(show_fn);
     Py_DECREF(msg);
 
@@ -654,7 +654,7 @@ warn_explicit(PyObject *category, PyObject *message,
     }
     else {
         text = message;
-        message = _PyObject_CallOneArg(category, message);
+        message = PyObject_CallOneArg(category, message);
         if (message == NULL)
             goto cleanup;
     }
@@ -997,7 +997,7 @@ get_source_line(PyObject *module_globals, int lineno)
         return NULL;
     }
     /* Call get_source() to get the source code. */
-    source = _PyObject_CallOneArg(get_source, module_name);
+    source = PyObject_CallOneArg(get_source, module_name);
     Py_DECREF(get_source);
     Py_DECREF(module_name);
     if (!source) {
@@ -1284,7 +1284,7 @@ _PyErr_WarnUnawaitedCoroutine(PyObject *coro)
     int warned = 0;
     PyObject *fn = get_warnings_attr(&PyId__warn_unawaited_coroutine, 1);
     if (fn) {
-        PyObject *res = _PyObject_CallOneArg(fn, coro);
+        PyObject *res = PyObject_CallOneArg(fn, coro);
         Py_DECREF(fn);
         if (res || PyErr_ExceptionMatches(PyExc_RuntimeWarning)) {
             warned = 1;