Changed | to union to support python 3.9

Author: ib-tjuhasz

infobip_api_client/configuration.py

@@ -121,7 +121,7 @@ def _ensure_https(host: Optional[str]) -> Optional[str]:
     def __init__(
         self,
         host: Optional[str] = None,
-        api_key: Optional[Dict[str, str] | str] = None,
+        api_key: Optional[Union[Dict[str, str], str]] = None,
         api_key_prefix: Optional[Dict[str, str]] = None,
         username: Optional[str] = None,
         password: Optional[str] = None,