fix mypy: ExecHelperTimeoutError is possible only with timeout set

(cherry picked from commit c8e9fdb)

Author: penguinolog

exec_helpers/subprocess_runner.py

@@ -161,7 +161,7 @@ def close_streams():  # type: () -> None
 
         wait_err_msg = _log_templates.CMD_WAIT_ERROR.format(result=result, timeout=timeout)
         self.logger.debug(wait_err_msg)
-        raise exceptions.ExecHelperTimeoutError(result=result, timeout=timeout)
+        raise exceptions.ExecHelperTimeoutError(result=result, timeout=timeout)  # type: ignore
 
     def execute_async(
         self,