Fix mypy warning

For some reason, I only get this warning when running in github actions.

Author: ondras12345

OMPython/OMCSession.py

@@ -289,7 +289,6 @@ def __init__(self,
         self._omc: Optional[Any] = None
         self._dockerCid: Optional[int] = None
         self._serverIPAddress = "127.0.0.1"
-        self._interactivePort = None
         self._temp_dir = pathlib.Path(tempfile.gettempdir())
         # generate a random string for this session
         self._random_string = uuid.uuid4().hex
@@ -437,6 +436,7 @@ def _set_omc_command(self, omc_path_and_args_list) -> list:
             extraFlags = []
         if self._docker:
             if sys.platform == "win32":
+                assert self._interactivePort is not None  # mypy complained
                 p = int(self._interactivePort)
                 dockerNetworkStr = ["-p", "127.0.0.1:%d:%d" % (p, p)]
             elif self._dockerNetwork == "host" or self._dockerNetwork is None: