How to write __getitem__ cleanly?

As much as it seems odd, I suspect that the way you have it is the best way to go about things. Patterns generally exist to encompass common use cases, but that doesn't mean that they should be taken as gospel when following them makes life more difficult. The main reason that PEP 443 gives for balking at explicit typechecking is that it is "brittle and closed to extension". However, that mainly applies to custom functions that take a number of different types at any time. From the Python docs on __getitem__:

For sequence types, the accepted keys should be integers and slice objects. Note that the special interpretation of negative indexes (if the class wishes to emulate a sequence type) is up to the __getitem__() method. If key is of an inappropriate type, TypeError may be raised; if of a value outside the set of indexes for the sequence (after any special interpretation of negative values), IndexError should be raised. For mapping types, if key is missing (not in the container), KeyError should be raised.

The Python documentation explicitly states the two types that should be accepted, and what to do if an item that is not of those two types is provided. Given that the types are provided by the documentation itself, it's unlikely to change (doing so would break far more implementations than just yours), so it's likely not worth the trouble to go out of your way to code against Python itself potentially changing.

If you're set on avoiding explicit typechecking, I would point you toward this SO answer. It contains a concise implementation of a @methdispatch decorator (not my name, but i'll roll with it) that lets @singledispatch work with methods by forcing it to check args[1] (arg) rather than args[0] (self). Using that should allow you to use custom single dispatch with your __getitem__ method.

Whether or not you consider either of these "pythonic" is up to you, but remember that while The Zen of Python notes that "Special cases aren't special enough to break the rules", it then immediately notes that "practicality beats purity". In this case, just checking for the two types that the documentation explicitly states are the only things __getitem__ should support seems like the practical way to me.

The antipattern is for code to do explicit type checking, which means using the type() function. Why? Because then a subclass of the target type will no longer work. For instance, __getitem__ can use an int, but using type() to check for it means an int-subclass, which would work, will fail only because type() does not return int.

When a type-check is necessary, isinstance is the appropriate way to do it as it does not exclude subclasses.

When writing __dunder__ methods, type checking is necessary and expected -- using isinstance().

In other words, your code is perfectly Pythonic, and its only problem is the error message (it doesn't mention slices).