Don't bother with checking the type. Use it as if it's the correct type, and if it's not, Vim will show a traceback from Python. It's the user's job to set the options correctly. We shouldn't crash of course, and won't; Vim will inform the user if there's a misconfiguration.

We might add explicit warnings for configuration options in the future if the user sets the wrong type, but if we do, it will be in the VimScript right after loading the module, not in the Python code.