make type hints compatible with py3.9

ur_py_ctl/ur_socket_client.py

 import logging
 import socket
-from typing import Union
+from typing import Optional, Union
 
 # TODO: Integration tests with socket server mimicking UR controller.
 
@@ -79,7 +79,7 @@ class URSocketClient:
 
     def send_script(
         self, script: Union[str, bytes], await_response=False
-    ) -> str | None:
+    ) -> Optional[str]:
         if isinstance(script, str):
             script = script.encode(encoding="utf-8")