Don't use Literal for tkinter widget options

[Akuli]

Fixes #4889

Using Literal in cget was the most controversial part of #4363. By instead making cget and __getitem__ behave in exactly the same way, we can put cget to Misc, the base class that all widgets inherit from. This is also how it's actually done in tkinter/__init__.py. Now any object with type Misc has a .cget() method as expected.

I also tried to do the same thing with the configure() method, but that's not perfect:

• There are two ways to call configure(), and I could only get one of them to work in Misc. The other one takes widget-specific keyword arguments, and I don't know what should correspond with that in the Misc base class. I tried **kw: Any, but that wasn't compatible with the widget-specific configure() methods with specific options.
• config is an alias for configure, and trying to add it to Misc (with def or with config = configure) creates mypy errors for some reason.

[Akuli]

Replacing these Literals with str shouldn't cause mypy to miss much more errors. By far the most common thing to do with a widget is changing an option with widget.config(option=value), and that's completely unaffected by this PR.