PyInstaller: Complete types in splash.pyi

[Sabfo]

No description provided.

[Sabfo]

Thanks for your review. I've updated the types for the requested attributes.
I think we can or even should simplify the type for script_name to str if you think that will be better for consistency and actually anyway Splash doesn't use and import into a user code.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[Avasam]

Thanks!