fix: use enum instead of backports

addresses https://developers.home-assistant.io/blog/2024/04/08/deprecated-backports-and-typing-aliases/

custom_components/hafas/config_flow.py

@@ -3,14 +3,14 @@
 
 import logging
 from typing import Any
+from enum import StrEnum
 
 from pyhafas import HafasClient
 from pyhafas.profile import DBProfile, KVBProfile, VSNProfile
 from pyhafas.types.fptf import Station
 import voluptuous as vol
 
 from homeassistant import config_entries
-from homeassistant.backports.enum import StrEnum
 from homeassistant.const import CONF_OFFSET
 from homeassistant.core import HomeAssistant
 from homeassistant.data_entry_flow import FlowResult