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/import.c

@@ -1206,7 +1206,7 @@ get_path_importer(PyThreadState *tstate, PyObject *path_importer_cache,
         PyObject *hook = PyList_GetItem(path_hooks, j);
         if (hook == NULL)
             return NULL;
-        importer = _PyObject_CallOneArg(hook, p);
+        importer = PyObject_CallOneArg(hook, p);
         if (importer != NULL)
             break;