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/codecs.c

@@ -147,7 +147,7 @@ PyObject *_PyCodec_Lookup(const char *encoding)
         func = PyList_GetItem(interp->codec_search_path, i);
         if (func == NULL)
             goto onError;
-        result = _PyObject_CallOneArg(func, v);
+        result = PyObject_CallOneArg(func, v);
         if (result == NULL)
             goto onError;
         if (result == Py_None) {
@@ -317,7 +317,7 @@ PyObject *codec_getstreamcodec(const char *encoding,
     if (errors != NULL)
         streamcodec = PyObject_CallFunction(codeccls, "Os", stream, errors);
     else
-        streamcodec = _PyObject_CallOneArg(codeccls, stream);
+        streamcodec = PyObject_CallOneArg(codeccls, stream);
     Py_DECREF(codecs);
     return streamcodec;
 }