[ModelicaSystemCmd] update definition of arg_set() - use Optional[]

Author: syntron

OMPython/ModelicaSystem.py

@@ -130,7 +130,7 @@ def __init__(self, runpath: pathlib.Path, modelname: str, timeout: Optional[int]
         self._timeout = timeout
         self._args = {}
 
-    def arg_set(self, key: str, val: str | dict = None) -> None:
+    def arg_set(self, key: str, val: Optional[str | dict] = None) -> None:
         """
         Set one argument for the executable model.