[py] Fix type annotation errors in service.py (#16250)

Co-authored-by: Navin Chandra <[email protected]>

Author: NoStory-py

py/selenium/webdriver/common/service.py

@@ -57,14 +57,15 @@ def __init__(
         driver_path_env_key: Optional[str] = None,
         **kwargs,
     ) -> None:
+        self.log_output: Optional[Union[int, IOBase]]
         if isinstance(log_output, str):
             self.log_output = cast(IOBase, open(log_output, "a+", encoding="utf-8"))
         elif log_output == subprocess.STDOUT:
-            self.log_output = cast(Optional[Union[int, IOBase]], None)
+            self.log_output = None
         elif log_output is None or log_output == subprocess.DEVNULL:
-            self.log_output = cast(Optional[Union[int, IOBase]], subprocess.DEVNULL)
+            self.log_output = subprocess.DEVNULL
         else:
-            self.log_output = log_output
+            self.log_output = cast(Union[int, IOBase], log_output)
 
         self.port = port or utils.free_port()
         # Default value for every python subprocess: subprocess.Popen(..., creationflags=0)