Currently, there's no PEP 484 way to type an Ellipsis, making it hard to add type checking to getitem/setitem. Type checkers (like MyPy, and it appears PyType) have supported "ellipsis" (as a string) as a stand-in for the missing type. This breaks linters like pyflakes, which expect "ellipsis" to be imported from somewhere even when it's a string, since it's not a builtin. (It MyPy's case, it also doesn't participate in narrowing, for some reason). It's also very difficult to discover.
It seems the most user-friendly solution would be to add a special case for Ellipsis, just like there is one for None (and NotImplemented might be due such a special case, too, based on the original thread from last year which I hope is linked in here). MyPy clearly differentiates between "..." and "Ellipsis" now; tuple[int, Ellipsis] does not work as a stand-in for tuple[int, ...]. Like None, there is only one Ellipsis, and it does not have an accessible base type.
If there's a problem with that solution, then adding "ellipsis" to typing would be a second possible solution. Then the name is correctly importable, and a bit more discoverable than a type-checker-only name. But IMO this should only be used if absolutely impossible to implement the first solution.
See also the issue in pyflakes: https://github.com/PyCQA/pyflakes/issues/616 with a couple of examples.