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/bytearrayobject.c

@@ -89,7 +89,7 @@ _canresize(PyByteArrayObject *self)
 PyObject *
 PyByteArray_FromObject(PyObject *input)
 {
-    return _PyObject_CallOneArg((PyObject *)&PyByteArray_Type, input);
+    return PyObject_CallOneArg((PyObject *)&PyByteArray_Type, input);
 }
 
 static PyObject *
@@ -2015,7 +2015,7 @@ bytearray_fromhex_impl(PyTypeObject *type, PyObject *string)
 {
     PyObject *result = _PyBytes_FromHex(string, type == &PyByteArray_Type);
     if (type != &PyByteArray_Type && result != NULL) {
-        Py_SETREF(result, _PyObject_CallOneArg((PyObject *)type, result));
+        Py_SETREF(result, PyObject_CallOneArg((PyObject *)type, result));
     }
     return result;
 }