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

Objects/fileobject.c

@@ -137,7 +137,7 @@ PyFile_WriteObject(PyObject *v, PyObject *f, int flags)
         Py_DECREF(writer);
         return -1;
     }
-    result = _PyObject_CallOneArg(writer, value);
+    result = PyObject_CallOneArg(writer, value);
     Py_DECREF(value);
     Py_DECREF(writer);
     if (result == NULL)