One doubt I have is whether we should change it so that slice[A] = slice[A, A, A] after all.

Without this convention, describing the most commonly used slice type, which I think is

slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex | None]

Probably requires an alias since it is so long.

Moreover, I think we need to clarify whether the collections.abc.Sequence protocol requires __getitem__ to support slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex | None], or if slice[int | None, int | None, int | None] is sufficient. This is likely a critical case as many users want to subclass Sequence, and it shouldn't be too inconvenient to type hint the implementation.

@AlexWaygood #13008 is the same as this, but defaults step to start | stop, so that slice[SupportsIndex | None] gives us the most commonly used slice type. In this case, people who want to use datetime/timedelta need to annotate with 3 values.

A decision needs to be made between:

1. Use this patch. For type hinting index-slices there are 3 options:
   1. Guide users to type hint slice[SupportsIndex | None], which means step will not be checked.
   2. Guide users to use an IndexSlice type alias provided by typing_extensions / typing.
   3. Guide users to use their own alias.
2. Use builtins.slice: more precise __new__ overloads and defaults for StopT and StepT. #13008, and guide users to annotate index-slices with slice[SupportsIndex | None], and users who want to use non-standard slices to provide all 3 type variables.

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

Closing in favor of #13008.